PART I: THE INDIVIDUAL ORGANISM WITH REGARD TO FORM AND METABOLISM
A. ELEMENTARY MORPHOGENESIS
The organism is a specific individual body, built up by a typical combination of specific and different parts, all of them endowed with specific physiological functions. It is implied in the words of this definition that the organism is different, not only from crystals, as already mentioned, but also from all combinations of crystals, such as those called dendrites and others, which consist of a typical arrangement of identical units, the nature of their combination depending on the forces of every single one of their parts. For this reason dendrites, in spite of the typical features in their combination, must be called aggregates; but the organism is not an aggregate even from the most superficial point of view.
We have said before, that the organism is not always the same in its individual life, that it has its development, leading from simpler to more complicated forms of combination of parts; there is a “production of visible manifoldness” carried out during development, to describe the chief character of that process in the words of Wilhelm Roux. We leave it an open question in our present merely descriptive analysis, whether or not there was already a “manifoldness”, in an invisible state, before development. In any case, there is a production of manifoldness as far as we can see even with our best instruments.
It certainly is of great importance to understand most clearly that there actually is a “production of visible manifoldness” with regard to form and to functions during ontogenesis in the descriptive sense; the knowledge of the fact of this process must be the very foundation of all studies on the theory of development in any case, and therefore we will devote this chapter to studies in merely descriptive embryology.
But descriptive embryology, even if it is to serve merely as an instance of the universality of the fact of visible “epigenesis”, can only be studied successfully with reference to a concrete case. We select the development of the common sea-urchin (Echinus microtuberculatus) as such a case, and we are the more entitled to select this organism rather than another, because most of the analytical experimental work, carried out in the interests of a real theory of development, has been done on the germs of this animal. Therefore, to know at least the outlines of the individual embryology of the Echinus may indeed be called the conditio sine qua non for a real understanding of what is to follow.
You are aware that all organisms consist of organs and that each of their organs has a different function: the brain, the liver, the eyes, the hands are types of organs in animals, as are the leaves and the pistils in plants.
You are also aware that, except in the lowest organisms, the so-called Protista, all organs are built up of cells. That is a simple fact of observation, and I therefore cannot agree with the common habit of giving to this plain fact the title of cell-“theory”. There is nothing theoretical in it; and, on the other hand, all attempts to conceive the organism as a mere aggregate of cells have proved to be wrong. It is the whole that uses the cells, as we shall see later on, or that may not use them: thus there is nothing like a cell-“theory”, even in a deeper meaning of the word.
The cell may have the most different forms: take a cell of the skin, of a muscle, of a gland, of the wood in plants as typical examples. But in every case two parts may be distinguished in a cell: an outside part, the protoplasm, and an inside part, the nucleus, to leave out of special account several others, which, by the way, may only be protoplasmatic modifications.
Protoplasm is a mere name for what is not the nucleus; in any case it is not a homogeneous chemical compound; it consists of many such compounds and has a sort of architecture; all organic functions are based upon its metabolism. The nucleus has a very typical structure, which stands in a close relation to its behaviour during the most characteristic morphological period of the cell: during its division. Let us devote a few words to a consideration of this division and the part the nucleus plays in it; it will directly bear on future theoretical considerations about development.
There is a certain substance in every nucleus of a cell which stains most markedly, whenever cells are treated with pigments: the name “chromatin” has been given to it. The chromatin always gives the reaction of an acid, while protoplasm is basic; besides that, it seems to be a centre of oxidation. Now, when a division of a cell is to occur, the chromatin, which had been diffusely distributed before, in the form of small grains, arranges itself into a long and very much twisted thread. This thread breaks, as it were by sections, into almost equal parts, typical in number for each species, and each of these parts is split at full length. A certain number of pairs of small threads, the so-called “chromosomes”, are the ultimate result of this process, which intentionally has been described a little schematically, the breaking and the splitting in fact going on simultaneously or occasionally even in reverse order. While what we have described is performing in the nucleus, there have happened some typical modifications in protoplasm, and then, by an interaction of protoplasmatic and nuclear factors, the first step in the actual division of the cell begins. Of each pair of the small threads of chromatin one constituent is moved to one side on the cell, one to the other; two daughter-nuclei are formed in this way; the protoplasm itself at the same time forms a circular furrow between them; the furrow gets deeper and deeper; at last it cuts the cell in two, and the division of the cell is accomplished.
Fig. 1.—Diagram of Cell-Division (after Boveri).
a. Resting cell; the chromatin distributed in the form of small granules inside the nucleus. Outside the nucleus is the “centrosome”, not mentioned in the text.
b. Beginning of division; the chromatin arranged in the form of a long thread. Centrosome divided in two.
c. The thread of chromatin cut into four parts, the “chromosomes”.
d. The four parts of the chromatin arranged symmetrically between the centrosomes and the star-like “spheres”.
e. Each of the chromosomes split at full length.
f. Beginning of division of protoplasm; the two parts of each chromosome separated.
g. End of cell-division.
Not only is the growth of the already typically formed organism carried out by a series of cell-divisions, but also development proper in our sense, as a “production of visible manifoldness”, is realised to a great extent by the aid of such divisions, which therefore may indeed be said to be of very fundamental importance (Fig. 1).
Each cell-division which promotes growth is followed by the enlargement of the two daughter-cells which result from it; these two daughter-elements attain the exact size of the mother-cell before division, and as soon as this size is reached a new division begins: so the growth of the whole is in the main the result of the growth of the elements. Cell-divisions during the first steps of embryology may behave differently, as will be described at a proper occasion.
The Egg: its Maturation and Fertilisation
We know that all the organs of an animal or plant consist of cells, and we know what acts a cell can perform. Now, there is one very important organ in all living beings, which is devoted to reproduction. This organ, the so-called ovary in animals, is also built up of cells, and its single cells are called the eggs; the eggs originated by cell-division, and cell-division is to lead from them to the new adult.
But, with a very few exceptions, the egg in the ovary is not able to accomplish its functions unless certain typical events have occurred, some of which are of a merely preparatory kind, whilst the others are the actual stimulus for development.
The preparatory ones are generally known under the name of “maturation”. The egg must be “mature”, in order that it may begin development, or even that it may be stimulated to it. Maturation consists of a rather complicated series of phenomena: later on we shall have occasion to mention, at least shortly, what happens in the protoplasm during its course; as to the nuclear changes during maturation it may be enough for our purposes to say, that there occur certain processes among the chromosomes which lead to an extension of half of them in the form of two very small cells, the “directive cells” or “directive or polar bodies”, as they have been somewhat cautiously called. The ripe or mature egg is capable of being fertilised. Before turning to this important fact, which, by the way, will bring us to our specially chosen type, the Echinus, a few words may be devoted to the phenomenon of “parthenogenesis”, that is to say, the possibility of development without fertilisation, since owing to the brilliant discoveries of the American physiologist, Jacques Loeb, this topic forms one of the centres of biological interest at present. It has long been known that the eggs of certain bees, lice, crayfishes, and other animals and also plants, are capable of development without fertilisation at all. Now, Richard Hertwig and T. H. Morgan already had shown that at least nuclear division may occur in the eggs of other forms—in the egg of the sea-urchin for instance—when these eggs are exposed to some chemical injuries. But Loeb2
succeeded in obtaining a full development by treating the eggs of echinoderms with chloride of magnesium; thus artificial parthenogenesis had been discovered. Later researches have shown that artificial parthenogenesis may occur in all classes of the animal kingdom, and may be provoked by all sorts of chemical or physical means. We do not know at present in what the proper stimulus consists that must be supposed here to take the place of fertilisation; it seems, of course, highly probable that it is always the same in the last resort.
But enough about processes, which at present are of a highly scientific but hardly of any philosophic interest.
By fertilisation proper we understand the joining of the male element, the spermatozoon or the spermia, with the female element, the egg. Like the egg, the spermatozoon is but a cell, though the two differ very much from one another in the relation between their protoplasm and nucleus: in all eggs it is the protoplasm which is comparatively very large, if held together with somatic cells; in the spermatozoon it is the nucleus. A large amount of reserve material, destined for the growth of the future being, is the chief cause of the size of the egg protoplasm. The egg is quite or almost devoid of the faculty of movement; while, on the contrary, movement is the most typical feature of the spermia. Its whole organisation is adapted to movement in the most characteristic manner: indeed, most spermatozoa resemble a swimming infusorium, of the type of Flagellata—a so-called head and a moving tail are their two chief constituents; the head is formed almost entirely of nuclear substance. It seems that in most cases the spermatozoa swim around at random, and that their union with the eggs is assured only by their enormous number; only in a few cases in plants have there been discovered special stimuli of a chemical nature, which attract the spermia to the egg.
But we cannot enter here more fully into the physiology of fertilisation, and shall only remark that its real significance is by no means clear.3
The First Developmental Processes of Echinus
Turning now definitively to the special kind of organism, chosen of our type, the common sea-urchin, we properly begin with a few words about the absolute size of its eggs and spermatozoa. All of us are familiar with the eggs of birds and possibly of frogs; these are abnormally large eggs, on account of the very high amount of reserve material they contain. The almost spherical egg of our Echinus only measures about a tenth of a millimetre in diameter; and the head of the spermatozoon has a volume which is only the four-hundred-thousandth part of the volume of the egg! The egg is about on the extreme limit of what can be seen without optical instruments; it is visible as a small white point. But the number of eggs produced by a single female is enormous and may amount to hundreds of thousands. This is one of the properties which render the eggs of Echinus so very suitable for experimental research; and, moreover, they happen to be very clear and transparent, even in later stages, and to bear all kinds of operations well.
The spermia enters the egg, and it does so in the open water—another of the experimental advantages of our type. Only one spermia enters the egg in normal cases, and only its head goes in. The moment that the head has penetrated the protoplasm of the egg a thin membrane is formed by the latter. This membrane is very soft at first, becoming much stronger later on; it is very important, for all experimental Work, that by shaking the egg in the first minutes of its existence the membrane can easily be destroyed without any damage to the egg itself.
And now occurs the chief phenomenon of fertilisation: the nucleus of the spermatozoon unites with the nucleus of the egg. When speaking of maturation, we mentioned that half of the chromatin was thrown out of the egg by that process: now this half is brought in again, but comes from another individual.
It is from this phenomenon of nuclear union as the main character of fertilisation that almost all theories of heredity assume their right to regard the nuclei of the sexual cells as the true “seat” of inheritance. Later on, we shall have occasion to discuss this hypothesis from the point of view of logic and fact.
After the complete union of what are called the male and the female “pronuclei”, the egg begins its development; and this development, in its first steps, is simply pure cell-division. We know already the chief points of this process, and need only add to what has been described that in the whole first series of the cell-divisions of the egg, or, to use the technical term, in the whole process of the “cleavage” or “segmentation”, there is never any growth of the daughter-elements after each division, such as we know to occur after all cell-divisions of later embryological stages. So it happens, that during cleavage the embryonic cells become smaller and smaller, until a certain limit is reached; the sum of the volumes of all the cleavage cells together is equal to the volume of the egg.
But our future studies will require a more thorough knowledge of the cleavage of our Echinus. The first division plane, or, as we shall say, the first cleavage plane, divides the eggs into equal parts; the second lies at right-angles to the first and again divides equally: we now have a ring of four cells. The third cleavage plane stands at right-angles to the first two; it may be called an equatorial plane, if we compare the egg with a globe; it also divides equally, and so we now find two rings, each consisting of four cells, and one above the other. But now the cell-divisions cease to be equal, at least in one part of the egg: the next division, which leads from the eight—to the sixteen-cell stage of cleavage, forms four rings, of four cells each, out of the two rings of the eight-cell stage. Only in one half of the germ, which we shall call the upper one, or which we might call, in comparison with a globe, the northern hemisphere, are cells of equal size to be found; in the lower half of the egg four very small cells have been formed at one “pole” of the whole germ. We call these cells the “micromeres”, that is, the “small parts”, on the analogy of the term “blastomeres”, that is, parts of the germ, which is applied to all the cleavage cells in general. The place occupied by the micromeres is of great importance to the germ as a whole: the first formation of real organs will start from this point later on. It is sufficient thus fully to have studied the cleavage of our Echinus up to this stage: the later cleavage stages may be mentioned more shortly. All the following divisions are into equal parts; there are no other micromeres formed, though, of course, the cells derived from the micromeres of the sixteen-cell stage always remain smaller than the rest. All the divisions are tangential; radial cleavages never occur, and therefore the process of cleavage ends at last in the formation of one layer of cells, which forms the surface of a sphere; it is especially by the rounding-up of each blastomere, after its individual appearance, that this real surface layer of cells is formed, but, of course, the condition, that no radial divisions occur, is the most important one in its formation. When 808 blastomeres have come into existence the process of cleavage is finished; a sphere with a wall of cells and an empty interior is the result. That only 808 cells are formed, and not, as might be expected, 1024, is due to the fact that the micromeres divide less often than the other elements; but, speaking roughly, of course, we may say that there are ten steps of cleavage-divisions in our form; 1024 being equal to 210.
We have learned that the first process of development the cleavage, is carried out by simple cell-division. A few cases are known, in which cell-division during cleavage is accompanied by a specific migration of parts of the protoplasm in the interior of the blastomeres, especially in the first two or first four; but in almost all instances, cleavage is as simple a process of mere division as it is in our sea-urchin. Now
Fig. 2.—Early Development of Echinus, the Common Sea-urchin.
a. Two cells.
b. Four cells,
c. Eight cells, arranged in two rings of four, above one another.
d. Sixteen cells, four “micromeres” formed at the “vegetative” pole.
e. Optical section of the “blastula,” a hollow sphere consisting of about one thousand cells, each of them with a small cilium.
the second step in development, at least in our form, is a typical histological performance: it gives a new histological feature to all of the blastomeres: they acquire small cilia on their outer side, and with these cilia the young germ is able to swim about after it has left its membrane. The germ may be called a “blastula” at this stage, as it was first called by Haeckel whose useful denominations of the first embryonic stages may conveniently be applied, even if one does not agree with most or perhaps almost all, of his speculations (Fig. 2).
It is important to notice that the formation of the “blastula” from the last cleavage stage is certainly a process of organisation, and may also be called a differentiation with regard to that stage. But there is in the blastula no trace of one part of the germ becoming different with respect to others of its parts. If development were to go on in this direction alone, high organisatory complications might occur: but there would always be only one sort of cells, arranged in a sphere; there would be only one kind of what is called “tissue”.
But in fact development very soon leads to true differences of the parts of the germ with respect to one another, and the next step of the process will enable us to apply different denominations to the different parts of the embryo.
At one pole of the swimming blastula, exactly at the point where the descendants of the micromeres are situated, about fifty cells lose contact with their neighbours and leave the surface of the globe, being driven into the interior space of it. Not very much is known about the exact manner in which these changes of cellular arrangement are carried out—whether the cells are passively pressed by their neighbours, or whether, perhaps, in a more active manner, they change their surface conditions; therefore, as in most ontogenetic processes, the description had best be made cautiously in fairly neutral or figurative words.
The cells which in the above manner have entered the interior of the blastula are to be the foundation of important parts of the future organism; they are to form its connective tissue, many of its muscles, and the skeleton. “Mesenchyme”, i.e. “what has been infused into the other parts”, is the technical name usually applied to these cells. We now have to learn their definite arrangement. At first they lie as a sort of heap inside the cell-wall of the blastula, inside the “blastoderm”, i.e. skin of the germ. But soon they move from one another, to form a ring round the pole at which they entered, and on this ring a process takes place which has a very important bearing upon the whole type of the organisation of the germ. You will have noticed that hitherto the germ with regard to its symmetry has been a monaxial or radial formation; the cleavage stages and the blastula with its mesenchyme were forms with two different poles, lying at the ends of one single line, and round this line everything was arranged concentrically. But now what is called “bilateral symmetry” is established; the mesenchyme ring assumes a structure which can be symmetrically divided only by one plane, but divided in such a way that one-half of it is the mirror image of the other. A figure shows best what has occurred, and you will notice (Fig. 3) two masses of cells in this figure, which have the forms of spherical triangles:
Fig. 3.—Formation of Mesenchyme in Echinus.
a. Outlines of blastula, side-view; mesenchyme forms a heap of cells at the “vegetative” pole.
a1. Heap of mesenchyme-cells from above.
b. Mesenchyme-cells arranged in a ring round the vegetative pole.
c. Mesenchyme-cells arranged in a bilateral-symmetrical figure; primordia of skeleton in the midst of two spherical triangles.
it is in the midst of these triangles that the skeleton of the larva originates. The germ had an upper and a lower side before: it now has got an upper and lower, front and back, right and left half; it now has acquired that symmetry of organisation which our own body has; at least it has got it as far as its mesenchyme is concerned.
We leave the mesenchyme for a while and study another land of organogenesis. At the very same pole of the germ where the mesenchyme cells originated there is a long and narrow tube of cells growing in, and this tube, getting longer and longer, after a few hours of growth touches the opposite pole of the larva. The growth of this cellular tube marks the beginning of the formation of the intestine, with all that is to be derived from it. The larva now is no longer a blastula, but receives the name of “gastrula” in Haeckel’s terminology; it is built up of the three “germ-layers” in this stage. The remaining part of the blastoderm is called “ectoderm”, or outer layer; the newly-formed tube, “endoderm”, or inner layer; while the third layer is the “mesenchyme” already known to us.
The endoderm itself is a radial structure at first, as was the whole germ in a former stage, but soon its free end bends and moves against one of the sides of the ectoderm, against that side of it where the two triangles of the mesenchyme are to be found also. Thus the endoderm has acquired bilateral symmetry just as the mesenchyme before; and as in this stage the ectoderm also assumes a bilateral symmetry in its form, corresponding with the symmetrical relations in the endoderm and the mesenchyme, we now may call the whole of our larva a bilateral-symmetrical organisation.
It cannot be our task to follow all the points of organogenesis of Echinus in detail. It must suffice to state briefly that ere long a second portion of the mesenchyme is formed in the larva, starting from the free end of its intestine tube; that the formation of the so-called “coelum” occurs by a sort of splitting off from this same original organ; and that the intestine itself is divided into three parts of different size and aspect by two circular sections.
But we must not, I think, dismiss the formation of the skeleton so quickly. I told you already that the skeleton has its first origin in the midst of the two triangular cell-masses of the mesenchyme; but what are the steps before it attains its typical and complicated structure? At the beginning a very small tetrahedron, consisting of carbonate of calcium, is formed in each of the triangles; the four edges of the tetrahedron are produced into thin rods, and by means of a different organogenesis along each of these rods the typical formation of the skeleton proceeds. But the manner in which it is carried out is very strange and peculiar. About thirty of the mesenchyme cells are occupied in the formation of skeleton substance on each side of the larva. They wander through the interior space of the gastrula—which at this stage is not filled with sea-water but with a sort of gelatinous material—and wander in such a manner that they always come to the right places, where a part of the skeleton is to be formed; they form it by a process of secretion, quite unknown in detail; one of them forms one part, one the other, but what they form altogether is one whole.
When the formation of the skeleton is accomplished, the
Fig. 4.—Larval Development of Echinus.
A. The gastrula.
B. Later stage, bilateral-symmetrical. Intestine begins to divide into three parts
S = Skeleton.
I = Intestine.
typical larva of our Echinus is built up; it is called the “pluteus” (Fig. 4). Though it is far from being the perfect adult animal, it has an independent life of its own; it feeds and moves about and does not go through any important changes of form for weeks. But after a certain period of this species of independent life as a “larva”, the changes of form it undergoes again are most fundamental: it must be transformed into the adult sea-urchin, as all of you know. There are thousands and thousands of single operations of organogenesis to be accomplished before that end is reached; and perhaps the strangest of all these operations is a certain sort of growth, by which the symmetry of the animal, at least in certain of its parts—not in all of them—is changed again from bilateral to radial, just the opposite of what happened in the very early stages!
But we cannot follow the embryology of our Echinus further here; and indeed we are the less obliged to do so, since in all our experimental work we shall have to deal with it only as far as to the pluteus larva. It is impossible under ordinary conditions to rear the germs up to the adult stages in captivity.
You now, I hope, will have a general idea at least of the processes of which the individual development of an animal consists. Of course the specific features leading from the egg to the adult are different in each specific case, and, in order to make this point as clear as possible, I shall now add to our description a few words about what may be called a comparative descriptive embryology.
Even the cleavage may present rather different aspects. There may be a compact blastula, not one surrounded by only one layer of cells as in Echinus; or bilaterality may be established as early as the cleavage stage—as in many worms and in ascidians—and not so late as in Echinus. The formation of the germ layers may go on in a different order and under very different conditions: a rather close relative of our Echinus, for instance, the starfish, forms first the endoderm and afterwards the mesenchyme. In many cases there is no tube of cells forming the “endoderm”, but a flat layer of cells is the first foundation of all the intestinal organs: so it is in all birds and in the cuttlefish. And, as all of you know, of course, there are very many animal forms which have no proper “larval” stage: there is one in the frog, the well-known “tadpole”, but the birds and mammals have no larvae; that is to say, there is no special stage in the ontogeny of these forms which leads an independent life for a certain time, as if it were a species by itself, but all the ontogenetical stages are properly “embryonic”—the germ is always an “embryo” until it becomes the perfect young organism. And you also know that not all skeletons consist of carbonate of calcium, but that there are skeletons of silicates, as in Radiolaria, and of horny substance, as in many sponges. And, indeed, if we were to glance at the development of plants also, the differences would seem to us probably so great that all the similarities would disappear.
But there are similarities, nevertheless, in all development, and we shall now proceed to examine what they are. As a matter of fact, it was especially for their sake that we studied the ontogeny of a special form in such detail; one always sees generalities better if one knows the specific features of at least one case. What then are the features of most general and far-reaching importance which may be abstracted from the individual history of our sea-urchin, checked always by the teachings of other ontogenies, including those of plants?
The First Steps of Analytical Morphogenesis
If we look back upon the long fight of the schools of embryologists in the eighteenth century about the question whether individual development was to be regarded as a real production of visible manifoldness or as a simple growth of visibly pre-existing manifoldness, whether, in the sphere of visibility, it was “epigenesis” or “evolutio”, there can be no doubt, if we rely on all the investigations of the last hundred and fifty years, that, taken in the descriptive sense, the theory of epigenesis is right. Descriptively speaking, there is a production of manifoldness in the course of embryology: that is our first and main result. Any one possessed of an average microscope may any day convince himself personally that it is true.
In fact, true epigenesis, in the descriptive sense of the term, does exist. One thing is formed “after” the other; there is not a mere “unfolding” of what visibly existed already, though in a smaller form; there is no “evolutio” in the old meaning of the word.4
Whether or not the theory of evolutio
may be maintained in a deeper sense of the word, will be discussed in a later chapter.
The totality of the line of morphogenetic facts can easily be resolved into a great number of distinct processes. We propose to call these “elementary morphogenetic processes”; the turning in of the endoderm and its division into three typical parts are examples of them. If we give the name “elementary organs” to the distinct parts of every stage of ontogeny which are uniform in themselves and are each the result of one elementary process in our sense, we are entitled to say that each embryological stage consists of a certain number of elementary organs. The mesenchyme ring, the coelum, the middle-intestine, are instances of such organs. It is important to notice well that the word elementary is always understood here with regard to visible morphogenesis proper and does not apply to what may be called elementary in the physiological sense. An elementary process in our sense is every distinct act of form-building, and an elementary organ is the result of every one of such acts.
The elementary organs are typical with regard to their position and with regard to their histological properties. In many cases, they are of a very clearly different histological type, as, for instance, the cells of the three so-called germ-layers; and in other cases, though apparently almost identical histologically, they can be proved to be different by their different power of resisting injuries or by other means. But there are not as many different types of histological structure as there are typically placed organs: on the contrary, there are many elementary organs of the same type in different typical parts of the organism, as all of you know to be the case with nerves and muscles. It will not be without importance for our future theory of development, carefully to notice this fact, that specialisation in the position of embryonic parts is more strict than in their histology.
But elementary organs are not only typical in position and histology, they are typical also with regard to their form and their relative size. It agrees with what has been said about histology being independent of typical position, that there may be a number of organs in an embryonic stage, all in their most typical positions, which, though all possessing the same histology, may have different forms or different sizes or both: the single bones of the skeleton of vertebrates or of adult echinoderms are the very best instances of this most important feature of organogenesis. If we look back from elementary organs to elementary processes, the specialisation of the size of those organs may also be said to be the consequence of a typical duration of the elementary morphogenetic process leading to them.5
I hardly need to say that the histology, form, and size of elementary organs are equally an expression of their present or future physiological function. At least they prepare for this function by a specific sort of metabolism which sets in very early.
The whole sequence of individual morphogenesis has been divided by some embryologists into two different periods; there is a first period, during which the foundations of the organisation of the “type” are laid down, and a second period, during which the histo-physiological specifications are modelled out (von Baer, Goette, Roux). Such a discrimination is certainly justified, if not taken too strictly; but its practical application would encounter certain difficulties in many larval forms, and also, of course, in all plants.
Our mention of plants leads us to another analytical result. If an animal germ proceeds in its development from a stage d to the stage g, passing through e and f, we may say that the whole of d has become the whole of f, but we cannot say that there is a certain part of f which is d, we cannot say that f is d + a. But in plants we can: the stage f is indeed equal to a + b + c + d + e + α in vegetable organisms; all earlier stages are actually visible as parts of the last one. The great embryologist, Carl Ernst von Baer, most clearly appreciated these analytical differences between animal and vegetable morphogenesis. They become a little less marked if we remember that plants, in a certain respect, are not simple individuals but colonies, and that among the corals, hydroids, bryozoa, and ascidia we find analogies to plants in the animal kingdom; but nevertheless, the differences we have stated are not extinguished by such reasoning. It seems almost wholly due to the occurrence of so many foldings and bendings and migrations of cells and complexes of cells in animal morphogenesis that an earlier stage of their development seems lost in the later one; those processes are almost entirely wanting in plants, even if we study their very first ontogenetic stages. If we say that almost all production of surfaces goes on outside in plants, inside in animals, we shall have adequately described the difference. And this feature again leads to the further diversity between animals and plants which is best expressed by calling the former “closed”, the latter “open” forms: animals reach a point where they are finished, plants never are finished, at least in most cases;
We have already said that the embryological process appears under the form of a production of visible manifoldness, and it will be important for what is to follow to analyse the concept of manifoldness a little more in detail.
We call manifold everything that is not elemental, but is a combination of parts. And in the first place, we may speak of a numerical manifoldness, that is, one which only refers to the number of elements in question. In this respect, the atom of iron is more manifold than the atom of hydrogen; for there are more electrons in the former.
But there may be a different manifoldness of construction in the realm of the same numerical manifoldness. A community of points which represents a flower has a higher degree of this sort of manifoldness than has the same community of points if arranged in squares, the relations between the points being more manifold in the first case than in the second.
Finally, there is what we shall call a manifoldness by rank, which may also have various degrees. A manifoldness of this sort is realised whenever a whole consists of parts which are wholes in themselves. A complicated engine is a good example of what we mean, and so also is the adult organism. The adult organism consists of organs, the organs of tissues, the tissues of cells, the cells of protoplasm, chromosomes, etc., these of molecules, the molecules of electrons and protons.
From our new points of view we may now say that, visibly, the embryological process produces a manifoldness of high rank out of a low rank manifoldness—an increase of numerical manifoldness, by assimilation, and of manifoldness of construction going hand-in-hand with this production.
This is also the ultimate foundation of the fact that embryology proceeds by steps or phases, i.e., that the adult is not formed out of the egg by one single act; or, in other terms, that embryology is a sequence of many single events in time.
All this will be very important in the course of our analytical work.
We have analysed our descriptions as far as we could, and yet we must confess that what we have found cannot be the last thing knowable about individual morphogenesis. There must be something deeper to be discovered: we only have been on the surface of the phenomena, we now want to get to the very bottom of them. Why then occurs all that folding, and bending, and histogenesis, and all the other processes we have described? There must be something that drives them out, so to say. But how shall we discover it?
Physicists always have used experiment and hypothetical construction. With these aids they have gone through the whole of the phenomena, and have erected the gigantic monument of modern physics.
It is the method of the physicists—not their results—that morphogenesis has to apply in order to make progress; and this method we shall begin to apply in our next chapters.
But before saying any more about the exact rational and experimental method in morphology, we first shall have to analyse shortly some general attempts to understand morphogenesis by means of hypothetic construction exclusively. For such attempts have become very important as points of issue for really exact research.