From the earliest times in which animals and plants formed objects of study a certain bifurcation in Biological Science is discernible which corresponds to two different points of view in relation to such objects. This cleavage of the Science into two divergent lines of advance has persisted throughout its history and has manifested itself in the division of Biology into several special departments each of which is connected with one or other of these two modes of regarding the organism. Many investigators have concerned themselves exclusively or mainly with one or other of these two divisions of the subject; and many others who have not exclusively cultivated either side of the subject have shown a tendency to emphasize the greater relative importance of one or other of the two aspects especially in relation to the problem of classification of animals and of plants. In the first of these modes of regarding the organism the attention is directed to the study of “form”; this is the point of view of the Anatomist; and the term Morphology invented by Goethe is now employed to designate the study of form in the most general sense; the term being not even always confined to the study of the forms of living organisms. Throughout the history of the Biological Sciences Morphology has advanced from the study of the forms of animals and plants as wholes to that of the structure connections and spatial positions of the various organs of which they are composed and their comparison in different animals or plants; then to the structure of the tissues; and in the nineteenth century to the study of the forms and structure of the cells of which the body of the organism is built up; and finally to the study of the chemistry of protoplasm consisting of a mixture of complex substances which is the fundamental material that distinguishes the living organism from non-living matter.
The Living Organism
Whilst Morphology has to do with the statical aspect of the organism it is the province of the other great division of the Biology of the single organism Physiology to deal with the kinetic aspect as expressed by “function.” Taking as its starting point the activities of an animal as a whole Physiology has advanced through the study of the functional activities of the various organs to that of the activities of the tissues; later to the study of the active life of the individual cells and finally to the metabolism of the protoplasm. It seems as if the two great departments of Morphology and Physiology had found their meeting point in the study of the protoplasm of which the structure and metabolism appear to represent the physical aspect of what we call life. To the department of Morphology there belongs that study of the anatomy and histology of extinct species which is denoted by the term Palaeontology. The study of Embryology that of the early stages in the growth of the organism its organs and tissues has belonged chiefly to Morphology but in recent times the Physiology of the processes at work during the development has also been studied. Both Embryology and Palaeontology are in close relation with those racial and evolutionary aspects of Biology of which I shall speak more directly in the two following lectures. The distinction between the Morphological and the Physiological points of view has exhibited itself in the history of the Science in relation to the problem of the classification of animals and plants. It is in fact clear that the element of arbitrariness which appertains to such classification leaves ample room for differences in the emphasis laid upon form and upon function in relation to the similarities and divergences upon which the classification is based.
Although there existed some knowledge both of the anatomical and of the functional aspects of animals and especially of man before the time of Aristotle (384–322 B.C.) as recorded in the writings of Hippocrates and his school Aristotle may be regarded as the founder of Comparative Anatomy. That great thinker and observer was much less of an Aristotelian and reached much more nearly a true conception of scientific method than the medieval thinkers who based their views upon his Philosophy. He had a knowledge of over 500 different animals and had an extensive acquaintance with the structure of many of them. He studied not only the more ordinary beasts birds and fishes but also cuttlefish snails oysters crabs crawfish lobsters sea-anemones sponges fish-lice and even intestinal worms. Extensive as was his anatomical knowledge his interest in it was however secondary to his interest in the functions of the various organs and parts of animals. He appears to have been the first to study in any detail the development of the chick; and he made a commencement of the study of comparative Embryology. The results of his study of form are contained in his great work the Historia Animalium but that work also contains the results of his observations in comparative physiology and in the distribution and behaviour of many species of animals. His later book De Partibus Animalium deals with what he regarded as the causes of the form and structure of animals and thus amounts to a discussion of the functions of their parts of the relations of form with function and of the adaptedness of structure. In the comparison of an animal of one species with that of another a clear distinction has been made in later times between homologies and analogies. By homology is understood the correspondence between the organs and parts of animals of different species in relation to a common spatial order; whereas analogy denotes a correspondence between organs and parts which have the same function in the two animals Aristotle made use of both these notions although he did not state quite clearly the distinction between them. He made the first attempt at a scientific classification of animals into a number of groups based upon similarities of structure. He distinguished between back-boned and back-boneless animals but held the erroneous view that only the former have blood. He recognized various modes of classifying animals not only in accordance with their structure but also in manners dependent on function such as their manner of life their mode of reproduction their food etc. Aristotle recognized clearly that animals belonging to the same one among the great groups into which he divided them have a unity in the plan of their construction. In his Historia Animalium taking man as the standard after describing his external and internal parts in detail he compared viviparous quadrupeds with man and traced out the unity of plan in the structure of man and all such quadrupeds. Although this constitutes a definite contribution to Morphology what he really sought for were not homologies but parts with the same functions; his interest being mainly in functioning organs and not in merely spatial relationship of parts. Aristotle foreshadowed various ideas and distinctions which came to be of great importance in Science in later times. Among these is the principle of division of labour amongst different organs a point which was emphasized in modern times by Milne-Edwards. Thus he writes in De Partibus Animalium: “Whenever therefore Nature is able to provide two separate instruments for two separate uses she does so instead of acting like a coppersmith who for cheapness makes a spit and lamp-holder in one.” He recognized the distinction between tissues and organs the homogeneous and heterogeneous parts of the body. Although he was unable to describe the structure of tissues as does the modern Histologist he described their distribution in the body. Aristotle also foreshadowed the notion of a scale of beings to the development of which in modern times I shall presently refer. It is of interest to observe that in Aristotle's view the gradation of organic forms is the consequence not the cause of gradation in their activities. Thus in the Historia Animalium he writes: Plants have no work to do beside nutrition growth and reproduction; they possess only the nutritive soul. Animals possess in addition sensation and the sensitive or perceptive soul.
Again he writes in De Partibus Animalium:
Plants again inasmuch as they are without locomotion present no great variety in their heterogeneous parts. For where the functions are but few few also are the organs required to effect them... Animals however that not only live but feel present a great multiformity of parts and this diversity is greater in some animals than in others being most varied in those to whose share has fallen not mere life but life in high degree. Now such an animal is man.
It thus appears that in Aristotle's teleological view the pre-determined character of the activities of organisms is to be regarded as the cause of their structural characteristics. Aristotle's Physiology was to some extent based upon observation but contained a large element of erroneous assumption. All the functions are connected with animal heat associated with the blood and centralized in the beating heart which is the seat of the soul. The brain is bloodless and produces mucus and the sense-organs are in the head so that they may not be overheated by the blood.
After the time of Aristotle and before the commencement of the modern epoch the only progress of importance in knowledge of Physiology was that made by the celebrated Physician Galen (132–200 A.D.) who recognized that the art of medicine should rest upon the basis of a physiological knowledge which must be dependent upon a groundwork of Anatomy. By his experiments on monkeys and swine he showed that the arteries contain blood not air; and he attained to an understanding of the meaning of the brain and nervous system. He was the first to show that the nerves connected with sensation are different from those which have to do with motion and that they form separate parts of the nervous system. He elaborated a pathological theory which dominated the theory and practice of medicine until the sixteenth century.
The inauguration of the modern period involving a breach with the dominant physiological tradition of Aristotle and Galen is due to Harvey (1578–1657). As in other branches of Science the fuller recognition of the importance of observation and experiment as the basis of all theory differentiates this period from the medieval. Harvey's introduction of precise methods of observing and experimenting into physiological Science had an importance in directing the attention of others to the true line of progress as great as his own great discovery of the circulation of the blood. The results obtained owing to the new impetus were collected and systematized by Albrecht von Haller (1708–1777) in his Elementa Physiologiae Corporis Humani. Among von Haller's own researches were those on respiratory movements the contractility of muscles and the irritability of nerves.
The origin of all modern mechanistic and physico-chemical theories of the living organism may be traced back to Descartes (1596–1650). The astronomical discoveries of Copernicus Tycho and Kepler the mechanics of Galileo and Harvey's discovery of the circulation of the blood suggested to him the idea that mechanical laws could be applied to the purpose of explaining the phenomena of life in the bodies of man and other animals. His Physiology was mechanistic in the strict sense and included no chemical conceptions; everything was made to depend upon heat hydraulics tubes and valves. He believed it possible to account on these lines for all the phenomena of organic life in animals and in man. His physiology was based upon that of Galen supplemented by Harvey's discovery of the circuital motion of the blood but he did not accept the idea that the heart acts as a propulsive apparatus. The food in the intestine was absorbed by the blood and carried to the liver where it became charged with the “natural spirits” and then passed to the heart which charged it with “vital spirits” in virtue of the innate heat of the heart and the action of the lungs. The blood became rarefied owing to the flame of the heart fed by the natural spirits; and this expansion of the fluid produced the circulation when directed by the valves of the heart and great vessels. The finer and rarefied parts of the blood pass off in two directions one to the organs of generation and the more important to the cavities of the brain where they not only serve to nourish that organ but also give rise to a fine ethereal flame or wind through the action of the brain upon them and thus form the “animal spirits.” From the brain these spirits are conveyed by means of the nerves which are regarded by Descartes as pipes to various parts of the body where they act upon the muscles. The impressions of the sense-organs are also conveyed by these tubular nerves to the brain.
In Descartes' view all animals except man are pure automata simple mechanisms devoid of any element of feeling or consciousness. “The animals” he says “act naturally by springs like a watch.” In the case of man there is added to the bodily mechanism the rational soul spiritualistic and immortal which is located in the pineal gland situated in the middle of the brain. By this assumption Descartes attempted to reconcile his mechanistic conceptions with his idealistic philosophy but it is safe to assert that he failed to give any intelligible account of the relations between the rational soul and the purely mechanistic body a failure which is characteristic of all the later theories of which hi may be regarded as the parent. In the latter part of the seventeenth century Physiology continued for some time to be purely mechanistic but in the modified sense that chemical discoveries were utilized in the description of the life of the organism.
The eighteenth century was a period in which vitalistic hypotheses dominated physiological conceptions especially owing to the influence of the chemical and vitalistic ideas of Stahl and his followers. During this time the progress of Physiology was much retarded by the lack of progress in Chemistry which was probably the result of Stahl's phlogistic hypothesis. Not until the great chemical discoveries of Lavoisier and his successors were adequate chemical conceptions made available in Physiological Science. Physico-chemical conceptions without vitalistic hypotheses became again dominant in Physiology and lasted throughout the nineteenth century culminating in the writings of Huxley and Max Verworn.
But little progress was made in the classification of animals during the eighteen centuries subsequent to the time of Aristotle. During the fifteenth and sixteenth centuries of our era considerable additions were made to the list of known animals but hardly any improvement was made in their classification. The first to make any such advance was John Ray (1628–1705) the predecessor of Linnaeus. He was the first to define the use of the term “species” as denoting a group of similar individuals exhibiting constant characteristics from one generation to another; and he was the first to emphasize the anatomical characteristics as a basis of classification. The greater systematizer Linnaeus (1707–1778) who has been described as “a classifying coordinating and subordinating machine” in his great work the Systema Naturae introduced a system of classification of plants and animals which formed the starting point of modern Systematics. Linnaeus employed a binomial system of nomenclature and graded his classification into classes orders genera species and varieties. He recognized six classes of animals—Mammals birds amphibians (including reptiles) fishes insects and vermes; this classification was afterwards made more precise by Lamarck who established sixteen classes instead of six. Linnaeus believed that each species was descended from a pair originally created each expressing an idea in the divine mind. Moreover in his doctrine of continuity taken in a loose sense of the term he maintained that the species could be arranged in series with no hiatus between two consecutive series. Thus unlike the present view he recognized no genetic relations between closely allied species and left no room for the occurrence of the discontinuous variations which many persons at the present time believe to be an important factor in evolution.
The advances made in human anatomy from the time of Aristotle until the end of the sixteenth century were accompanied by but little advance in the knowledge of comparative Anatomy. A great impetus was given to anatomical studies by the invention of the microscope at the beginning of the seventeenth century. One of the first effects of the use of the microscope was the discovery of the complex structure of tissues. Up till that time they had been regarded as little more than inorganic substances possessing however some organic properties such as contractility. Thus the study known as Histology came into being and in particular important discoveries relating to muscle fibres were made. Among other applications of the microscope was the study of the comparative Anatomy of lower animals and that of the metamorphoses of insects with which the name of Swammerdam is associated. One of the first to make extensive use of the new instrument was Malpighi who studied by its aid the development of the chick. Pie also ascertained the minute structure of the lungs and demonstrated the connection of the arteries with the veins; further he described the histology of the spleen the kidney the liver and the cortex of the brain; he showed that the liver is really a conglomerate gland and discovered the Malpighian bodies in the kidney.
The biological studies of the eighteenth century were largely in the direction of general natural history and under the influence of Linnaeus of the problem of classification. The minute study of insects was continued by Réaumur and Bonnet who attached however more importance to their habits and physiology than to their anatomy. The general conception to which I have already alluded of a scale of beings was first put into a detailed and systematic form by Bonnet (1720–1793) and was in fact extended by him to all the objects in the Universe. He constructed a long table headed “Idée d'une Echelle des etres naturels” which begins with Man the Orang-outang the Ape Quadrupeds and ends with earth water fire and more subtle matter. The scale is not based upon any definite principle either morphological or functional but is supposed to represent a gradation involving all possible orders of perfection. This conception of a gradation of beings was also held by Buffon (1707–1788) but in his hands it takes more consistent form as a functional gradation. He pointed out the fact that the groups of Invertebrates are very different in structural plan from those of the Vertebrates and had a clear conception of the unity of plan which is characteristic of the Vertebrates. Moreover he for the first time expressed the idea that community of origin might be at the base of the unity of plan although he was far from being a consistent Evolutionist. He pointed out the difficulty of supposing that one species may arise from another by a process of degeneration and oscillated between the ideas that species are definitely discontinuous with one another and that they can be united in larger groups.
Xavier Bichat (1771–1802) who was mainly a human anatomist worked out in detail the Aristotelian distinction between the animal and the vegetative parts and functions of animals to which Buffon had also drawn attention. The animal life which does not appertain to plants he described as the order of functions which connect the animal with its environment; these organs are the afferent and efferent nerves the brain the sense-organs and the voluntary muscles; the brain being the central organ. The organic or vegetative life has for its central organ the heart and includes the processes of digestion circulation respiration exhalation absorption secretion nutrition and calorification. He regarded the plant and animal as standing for two different modes of life. The only relation which the plant has with the environment is that involved in nutrition; the animal has in addition to this organic life a life of active relation with surrounding things. He observes1 that:
One might almost say that the plant is the framework the foundation of the animal and that to form the animal it sufficed to cover this foundation with a system of organs fitted to establish relations with the world outside. It follows that the functions of the animal form two quite distinct classes. One class consists in a continual succession of assimilation and excretion; through these functions the animal incessantly transforms into its own substance the molecules of surrounding bodies later to reject these molecules when they have become heterogeneous to it. Through this first class of functions the animal exists only within itself; through the other class it exists outside; it is an inhabitant of the world and not like the plant of the place which saw its birth. The animal feels and perceives its surroundings reflects its sensations moves of its own will under their influence and as a rule can communicate by its voice its desires and its fears its pleasures and its pains. I call organic life the sum of the functions of the former class for all organized creatures plants or animals possess them to a more or less marked degree and organized structure is the sole condition necessary to their exercise. The combined functions of the second class form the “animal” life so named because it is the exclusive attribute of the animal kingdom.
Bichat contrasts the symmetry of the nerves and muscles of the animal life with the asymmetrical arrangement of the visceral muscles and the sympathetic nerved which belong to the organic life. He points out that habit is all-important in the animal life but denies that habit has any influence upon the organic life. He states that the organs of the organic life attain their full perfection independently of use; whereas the organs of the animal life require education in order to reach perfection. These views as to the independence of the organic life of habit and use would probably no longer be accepted without considerable modification.
A very similar distinction between animal and vital functions was emphasized by the great Comparative Anatomist Cuvier (1769–1832) who studied both structure and function and even regarded the latter as the more important in that it determined the former. Following Aristotle he asserted that a plant is an animal that sleeps. He was aware owing to the recent progress of Chemistry that the material of the body is principally composed of combinations of Carbon Nitrogen Hydrogen and Phosphorus forming albumen fibrine etc. Although the discovery of the cellular nature of tissues was not made until after his death he observed that the organism can be resolved into small flakes and filaments which form a “cellulosity.” Cuvier was the first completely to recognize as a definite principle the harmony between structure and function. “It is” he writes “on this mutual dependence of the functions and the assistance which they lend one another that are founded the laws that determine the relations of their organs; these laws are as inevitable as the laws of metaphysics and mathematics for it is evident that a proper harmony between organs that act one upon another is a necessary condition of the existence to which they belong.” We have here an attempt to form a concept of the coordinated organism as distinct from the sum of the parts considered either in relation to structure or function; but by the conditions of existence he meant adaptations of functions and organs within the organism and he hardly considered the external conditions or the environment. In accordance with his well-known principle of correlation from one part of an animal having given a model of the group to which it belongs the whole may be constructed for as he says: “All the organs of an animal form a single system the parts of which hang together and act and react upon one another; and no modifications can appear in one part without bringing about corresponding modifications in all the rest.” From the shape of one organ the shape of the other organs can be inferred having given sufficiently extensive knowledge of functions and of the relation of structure to function in each kind of organ. The functional dependence of the parts he interpreted in terms of what is later known as the general metabolism of the organism that is the constant chemical changes in all the parts and the accompanying interchanges with the outside.
One of the most important results of Cuvier's work is his division of the animal kingdom into four principal types of form the Vertebrates Molluscs Articulates and Radiates. The first three have bilateral symmetry and the last radial symmetry. In formulating these four divisions each of which is built upon one plan Cuvier was influenced by the idea that the characters of the two sets of organs the vegetative and the animal and the correlations within each must form the basis of the classification.
In contradistinction with Cuvier it was held by his adversary Geoffroy Saint-Hilaire (1772–1844) that the structure of all animals may be referred to a single type. He went so far as to assert that “There is philosophically speaking only a single animal.” By that single animal he meant an abstract generalized type to which by the principle of homology all actual animal structures could be made to conform. With him homology was not so much homology of organs as of parts and connections His principle of connections was his guide in tracing an organ through all its functional transformations for as he says: “an organ can be deteriorated atrophied annihilated but not transposed.” As a pure Morphologist for whom “form” was everything his difference from Cuvier's attitude in which the stress is laid on “function” was fundamental. His attempts to prove by means of what appear to be very far-fetched homologies that animals as far apart as Vertebrates and Cephalopods conform to the same fundamental type were criticized and demolished by Cuvier. Cuvier showed that although Saint-Hilaire had discovered many hidden homologies especially by his important discoveries concerning foetal structure the unity of plan and composition as conceived by Saint-Hilaire does not exist in actuality. Cuvier further maintained that the whole principle of homology so far as it is valid is subordinate to the principle of the functional coordination and adaptation of the parts.
Biologists in Germany and also to a large extent in France were during the early part of the nineteenth century under the influence of a number of ideas which formed part of what is known as the Philosophy of Nature. The principal conceptions of this school were the existence of a unique plan of structure the idea of the scale of beings and the notion of parallelism between the stages of individual development and the stages of the scale of beings which latter has for us an obvious connection with theories of evolution. A further theory of this school was that of the repetition or multiplication of parts within the individual of which the vertebral theory of the skull is the most striking example. The law of parallelism first laid down by Kielmeyer and afterwards in a more developed form by Oken (1779–1851) asserted that the embryo of every animal passes during its development through all stages of the animal kingdom or at least through the stages of one or more classes lower down in the scale. It was held that the animal kingdom is a dismemberment of the highest animal man and thus that animals are only the persistent foetal stages or conditions of man who contains within himself all the animal kingdom. The embryo of higher animals was compared with the adults of lower animals. It was stated for example by Tiedemann that “Every animal before reaching its full development passes through the stage of organization of one or more classes lower in the scale or every animal begins its metamorphosis with the simplest organization.” A detailed account of facts which support this theory was given by Meckel but his treatment contains very imaginative comparisons between organs of animals of widely differing groups and involves a mixture of morphological homologies with physiological analogies. Meckel admitted that man does not pass in his development through the whole animal series but asserted that at least as regards single organs or organ-systems the embryo of man passes through many animal stages. Although he was not a thoroughgoing evolutionist he held that the higher animal in his gradual evolution passes through the permanent organic stages which lie below it. As he says: “The development of the individual organism obeys the same laws as the development of the whole animal series.” An adherent of this school K. G. Cams asserted as a general law of Nature that the higher formations include the lower; that the animal includes the vegetable; that it is by a rational necessity that the development of a perfect animal repeats the series of antecedent formations. The theory of the repetition or multiplication of parts within the organism was pushed to the absurd extreme of attempting to demonstrate that the whole organization is repeated in certain of its parts; for example Oken asserted that in the head the whole trunk is repeated the upper jaw corresponding to the arms the lower to the legs and that in each jaw the same bony divisions exist as in the limbs. Nevertheless the recognition of serial homologies was a real contribution to morphology although many absurd or arbitrary homologies were maintained. It was maintained by Carus that the whole skeleton is only a repeated vertebra. The influence of the Philosophy of Nature as an a priori theory is indicated in the statement of Carus that in respect of the formation of the skeleton throughout the animal kingdom he wishes to know “how such and such a formation is realized in virtue of the eternal laws of reason.” He held that all forms of skeletons can be deduced from the hollow sphere so that every skeleton can be represented schematically by a number of hollow spheres suitably modified in shape and suitably arranged; and he endeavoured to work out this idea as applied both to vertebrates and invertebrates. Although he was strongly under the influence of the a priori ideas of the German anatomists of the period he was in many respects less a disciple of Saint-Hilaire than of his great opponent Cuvier; he held that the connections of bones and muscles change in accordance with functional requirements and he did not accept the “law of connections” in its rigorous form.
The leading ideas of Cuvier in relation to functions of Saint-Hilaire in relation to the principle of connections and Oken's notion of the serial repetition of parts were combined modified and reduced to clearer forms by Richard Owen. The main idea developed in his work On the Archetype and Homologies of the Vertebrate Skeleton is that the vertebrate skeleton is composed of a series of segments each of which he calls a vertebra. His archetype is a scheme of what is usually constant in the vertebrate skeleton both the number and arrangement of the bones in any actual vertebrate being subject to variation. He defined the object of his work to be to deduce the relative value and constancy of the different vertebral elements and to trace the kind and extent of their variations within the limits of a plain and obvious maintenance of a typical character. He accepted in a modified form Oken's vertebral theory of the skull which was afterwards demolished by Huxley. In the determination of homologies he followed Saint-Hilaire's principle of connections and rejected the method of their determination by the mode of development. In his view comparative anatomy explains embryology and not the reverse. Relations of homology he analysed into three kinds special general and serial homology. Special homology consists of the correspondence of a special part or organ determined by its relative position and connections with a part or organ in a different animal; thus involving reference to a common type. Owen's general view of the nature of living things was that organic forms result from the competitive working of two principles the first of which brings about a vegetative repetition of structure while the other involving a teleological factor shapes the living organism to its functions. The first of these principles “a general polarizing force” illustrating the archetype of the vertebrate skeleton is the same principle which produces repetition of the forms of crystals in the inorganic domain. The second principle the “adaptive” or “special organizing force” produces the diversity of organic beings.
Although observations of the development of the embryo had been made from the time of Aristotle Embryology attained its position as a Science in the hands of von Baer (1792–1876). The first volume of his great treatise on the subject published in 1828 contains a full and adequate account of the development of the chick as obtained by minute and accurate observations and a discussion of the laws of development in general. In the Scholia at the end of his description of the development of the chick he refutes the notion of pre formation; and defends the idea that the essential nature of the animal determines its differentiation and that no stage of development is solely determined by the antecedent stage. He holds the vitalistic conception that a guidance involving the idea of the completed whole is active at each stage of development. He shows that the process of the development of the embryo is one of differentiation by which the germ becomes increasingly individualized and the developing animal increasingly independent. In describing the stages of development he lays down his theory of germ-layers. In his account of the process he states that first of all the germ separates out into heterogeneous layers which with advancing development acquire ever greater individuality and on their first appearance show rudiments of the structures which will characterize them later. In the germ of the bird at the beginning of incubation there can be distinguished an upper smooth continuous surface and a lower more granular surface. The blastoderm then separates into two layers of which the lower develops into the plastic body-parts of the embryo the upper into the animal parts; the lower shows clearly a further division into two subsidiary layers the mucous-layer and the vessel-layer; the original upper layer also shows a division into two which form respectively the skin and the muscle-layer the latter of which contains in an undifferentiated state the skeletal and muscular systems the connective tissues and the nerves belonging to them. Thus the process of determination results in the formation of four layers. The uppermost layer will form the outer covering of the embryo; from it there differentiates out at an early stage the rudiment of the central nervous system forming a more or less independent layer. Below this outermost layer is the one from which the muscular and skeletal system is developed then the vessel-layer which gives rise to the main blood vessels. The innermost layer will form the mucous membrane of the alimentary canal and its dependencies; it is originally like the other layers a flat plate. From all these layers tubes are developed by the bending round of their edges; the process of primary differentiation is then complete. He then describes the later histological and other morphological differentiations which occur concurrently. Through morphological differentiation the various parts of the fundamental organs become specialized through unequal growth first into the primitive organs and then into the functional organs of the body. At the same time there is a differentiation in the substance of the layers whereby cartilage muscle and nerve separate out and a part of the mass becomes fluid. Through histological differentiation the texture of the layers and incipient organs becomes individualized. Originally the germ consists of an almost homogeneous mass with clear or dark globules in suspension. This homogeneity gradually gives place to heterogeneity the structureless mass becomes fibrous to form muscles hardens to form cartilage and bone liquifies to form blood and differentiates in a large number of other ways.
Von Baer showed that the extreme views of those who were dominated by the law of parallelism as stated by Meckel and Serres that the higher animals repeat in their development the adult stages of the lower went far beyond the facts. He pointed out for example that the developing chick is at a very early stage demonstrably a vertebrate and does not recapitulate the organization of a polyp a worm or a mollusc. By means of various examples he showed that the recapitulation is never that of the whole organization of a lower animal but only that of particular parts or simple organs. Like Cuvier von Baer recognized four main types of animals the vertebrate the molluscan the longitudinal and the radiate. He held that the manner of development is determined by the type of organization; the type being observable in the very earliest stages of development In his view the development of the individual is always from the general to the special; the general characters of the group to which an embryo belongs appear earlier in development than the special characters of the particular animal. There is during the development a progressive gradation of generality the less general structural features always appearing after the more general. So far from endorsing Meckel's law of Parallelism he held that the embryo instead of passing through the states of other definite forms separates itself from them; and that the embryo of a higher animal never resembles the adult of another animal form but only its embryo. Thus not even within the group is there a real scale which the higher forms must mount. The apparent resemblance between the embryos of the higher more differentiated members of the group with the lower less differentiated adult forms arises from the fact that these latter diverge but little from the generalized type. Embryology is of great assistance to comparative anatomy of which the aim is to discuss the general type or the common plan of structure of the animals of a group. For as the embryo develops from the general to the special the state in which each organ first appears should represent the typical state of that organ within the group. Thus the true homologies of parts will be best determined by studying their earliest developments. Homologies should be restricted to a single type; organs with similar functional relations belonging to members of different types should not be regarded as homologous on account of their different modes of development. The classification should not depend on the adult structure but on the characters shown in early development because these latter show the characters of the type to which an animal belongs in their more generalized form.
Von Baer like Cuvier rejected the notion of the scale of beings not only by recognizing the existence of four totally different types but by showing that even within one type any serial arrangement only held good for separate organs or sets of organs and not for the whole complex of organs; so that there is no actual scale of beings even within one and the same type. It has been said that man is only the highest animal in respect of his nervous system.
The publication by Theodor Schwann in 1839 of the discovery that the tissues in the animal body are composed of cells constituted a most notable and far-reaching advance upon the discovery of the complex structure of the tissues. In accordance with the great conceptual scheme the cell theory the cell is both a morphological or statical unit and also a physiological or dynamical unit in the formation of the complete body. It is a statical unit inasmuch as all plants and animals are built up of cells and modifications of cells. It is a dynamical unit inasmuch as the life of the organism is dependent upon the activities of the cells of which it is composed. In the words of Schwann: “The whole organism subsists only by means of the reciprocal action of the single elementary parts.” It was in the domain of Botany that the existence of cells was first observed but Schwann was the first to announce the identity of the cellular structure of all living beings animal or vegetable. In the seventeenth century cells in plants had been discovered by Robert Hooke Malpighi and Leeuenhoek but they were then not regarded as living independent structural units. In the nineteenth century when great improvements in the microscope had been made it was established by the work of various observers that the tissues of plants are composed of elements which can be reduced for the most part to spherical closed cells. By the coalescence and elongation of the cells the vessels and fibres of the plants are formed. At first attention was concentrated on the cell-walls but in 1838 Schleiden pointed out the importance of a discovery that had been made a few years earlier of the existence in every cell of a small body which he called the cytoblast and which is now usually called the nucleus; this nucleus is embedded in a gummy substance called the cytoplasm. Schleiden showed that plants are built up of cells and their modifications and that a single cell or ovum is the original form of the plant embryo; further he gave a theory of the mode in which by cell division new cells are formed. The conception that the cell is a partially independent living unity was expressed by him in the statement that “Each cell carries on a double-life; one a quite independent and self-contained life the other a dependent life in so far as the cell has become an integral part of the plant.” Here we have in a distinct form the view in relation to the plant organism that it is to be regarded as a multiplicity in unity a notion which was extended by Schwann to the animal organism. The occurrence of small globules in animal tissues had been observed earlier for example by von Baer in the embryo of the chick; and that cells were to be found in animal tissues was well known to Johannes Müller and other investigators but a complete theory of cells in animal tissues was the work of Schwann1. In the first part of his book Microscopic Investigations Concerning the Agreement in Structure and Growth of Animals and Plants he dealt with cartilage cells and with the cells of the noto-chord showing that the nucleus the nucleolus and the cell-walls play the same part and behave in the same manner as in the plant cells in accordance with Schleiden's theory. It was already known that all animals are developed from an ovum; the fundamentally important discovery of the existence of the ovum in mammals had been made by von Baer in 1827. The position of the ovum in relation to the cell theory was a subject which Schwann carefully investigated; he came to the conclusion that the ovum is itself a cell but not one of the simplest kind. Every individual organism begins its life as a single cell; and in the simplest or uni-cellular organism such as the Amoeba it remains uni-cellular; in other cases the gradual formation of the body is due to the division and successive re-division of the fertilized ovum into a coherent mass of cells. Although there are considerable differences between the cells in any one organism and in the cells of different organisms there appears to be a considerable amount of agreement in the structure of all cells of animals and plants. The cell theory has been the basis of the modern theories of heredity of which I shall speak in the next lecture. The study of the cleavage of the fertilized ovum and the gradual formation of cells by segmentation leading to the formation of tissues was commenced by Schwann and carried on by various investigators of whom von Kölliker was the most eminent on account of the vast amount of detailed embryological and anatomical work he accomplished. Although the cell theory has been for the most part accepted as a representation of the facts a certain amount of criticism has been directed against its all-sufficiency based upon some exceptional cases which have been observed in which cell-walls appear to be absent the nuclei dividing without cellular division the result being a mass of protoplasm containing many nuclei but without cell boundaries. The fact that both in animals and plants intercellular filaments composed of protoplasm have been discovered connecting the cells may also not be without significance. The complete generality of the process of cell-formation in organic life has been challenged; and that process has been held by some investigators such as Sachs on the botanical side and Sedgwick on the zoological side to be a secondary process the growth of the protoplasmic mass itself being the primary factor in the development of the plant or animal. It has in fact been held that the plant forms cells but the cells do not form plants. However this may be the cell theory as a working scheme has been of inestimably great assistance in the development of biological science.
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