Morphogenesis

“ Owing to this composite character of the cross-sections, taken as elements of harmonious systems, a special phenomenon of morphogenesis is presented to us, which teaches somewhat more than the mere concept of harmonious-equipotentiality can express. If composite elements concerned in morphogenesis result in one whole organisation in spite of the development of the single tissues of these elements going on independently, then there must be a sort of correspondence or reciprocity of the harmonious development among these tissue constituents themselves ”

“ Almost all that has been investigated of the part played by the external conditions of development has little bearing on specific morphogenesis proper, and therefore may be left out of account here: we must, however, lay great stress on the general fact that there is a very close dependence of morphogenesis on the outside factors, lest we should be accused afterwards of having overlooked it. ”

“ Entelechy has ruled the individual morphogenesis of the generation which is regarded as being the starting-point for inheritance, and will rule also the morphogenesis of the generation which is to follow; entelechy determines the egg to be what it is, and the morphogenesis starting from this egg to be what it is also. ”

“ As for our second proof of vitalism, stating that no kind of machine inside the germ-cells can possibly be the foundation of their morphogenesis, it is clear that the protoplasm and the nucleus may both come into account here on equal terms. ”

“ At this stage we leave for a while our analytical studies of ontogeny proper. We must not forget that typical ontogenesis is not the only form in which morphogenesis can occur: the organic form is able to restore disturbances of its organisation, and it certainly is to be regarded as one of the chief problems of analytical morphogenesis to discover the specific and real stimulus which calls forth the restoring processes. ”

“ 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. ”

“ Let us begin our studies of adaptations in a field which may justly be called a connecting link between morphogenesis and physiology proper, not yet wholly separated from the science of the organic form, morphology. ”

“ It appears to me probable that more insight will be gained into the real nature of life and organisation by concentrating on the active response of the animal, as manifested both in behaviour and in morphogenesis, particularly in the post-embryonic stages, than by giving attention exclusively to the historical aspect of structure, as is the custom of "pure morphology." ”

“ The chief result of the first main part of our lectures has been to prove that an autonomy of life phenomena exists at least in some departments of individual morphogenesis, and probably in all of them; the real starting-point of all morphogenesis cannot be regarded as a machine, nor can the real process of differentiation, in all cases where it is based upon systems of the harmonious equipotential type. ”

“ Morphological adaptation is no part of individual morphogenesis proper, but occurs at the end of it; at least it never occurs previous to the full individual life of an organism, previous to its true functional life ”

“ On Cell-division.—We have already said a good deal about the importance of cell-division in ontogeny: it accompanies very many of the processes of organisation in all living beings. But even then, there are the Protozoa, in the morphogenesis of which it does not occur at all, and there have also become known many cases of morphogenesis in higher animals, mostly of the type of regulation, in which cellular division is almost or wholly wanting. ”

“ But the harmony revealed to us in every typical morphogenesis, be it normal or be it regulatory, seems to forbid us to connect transformism with the means of morphogenesis. And therefore we shall close this discussion about the most problematic phenomena of biology with the declaration, that we regard it as more congruent to the general aspect of life to correlate the unknown principle concerned in descent with entelechy itself, and not with its means. ”

“ Now, you know that all Protozoa are but one highly organised cell: we have therefore here an instance where the so-called “elements” of our harmonious-morphogenetic system are not cells, but something inside of cells; and this feature must appear to be of very great moment, for it first shows, as we have already pointed out on another occasion, that morphogenesis is not dependent on cell-division, and it states at the same time that our concept of the harmonious-equipotential system may cover a very great area—that, in fact, it is a scheme of a very wide extent. ”

“ Why the cells grow into the hollow sphere and not into the opposite direc­tion is unknown. The next step is the forma­tion of a skeleton by the forma­tion of crystals consisting of the CaCO3 by the mesenchyme cells surrounding the intestine. For the establishment of the principle in which we are interested the descrip­tion of morphogenesis need not be carried farther.
This principle which is under discussion here is the development of a purposeful arrangement of organs out of the egg. If we assume that the egg consists of homogeneous material we are indeed confronted with a riddle. ”

“ Homology is still determined by position, by connections, in the embryo as in the adult. "Similarity of development" has become the criterion of homology in the eyes of the embryologist, but "similarity of development" means, not identity of histological differentiation, but similarity of connections throughout the course of development. For the purposes of morphology, development has to be considered as an orderly sequence of successive forms, not in its real nature as a process essentially continuous. Morphology has to replace the living continuity by a kinematographic succession of stages. ”

“ But each unicellular organism would in general inherit the peculiarities, and repeat the life-history of its grandparent-cell, by dividing into two adherent cells. A race of two-celled organisms would thus be established. We may fairly believe that in time a second variation, which also proved fortunate, occurred, whereby the four grand-daughter cells remained adherent until reproduction; and afterwards other variations of the same nature, till an organism was at length evolved which consisted of a multitude of cells adherent together for the common benefit—the morula. ”

“ When the cells of the morula stage of an embryo are shaken asunder, each, instead of forming the appropriate part of a single organism, may form a complete new organism. And similarly in the development of a complicated organism, the suppression or doubling of a single cell or group of cells may bring about striking differences in the symmetry of the adult, or the reduction or increase in the number of metameric organs. A slight change in the structure or activity of a gland, by altering the internal secretion, may produce widespread alterations even in an adult organism ”

“ Section of the egg of the beetle, Calligrapha, showing the pigment at one end where the germ cells will later develop as shown in the other two figures. (After Hegner.) At every division of the cell the chromosomes also divide. Half of these have come from the mother, half from the father. Every cell contains, therefore, the sum total of all the chromosomes, and if these are the bearers of the hereditary qualities, every cell in the body, whatever its function, has a common inheritance. ”

“ But some of the hereditary units in each cell produced are active. They multiply, grow and migrate out of the nucleus, and get among the units of the enveloping protoplasm. During this activity they undergo physical and chemical changes and effect corresponding differentiations in the protoplasm of the cell. Thus, through many mitoses and many differentiations of the protoplasm of cells, we finally derive from the fertilized ovum all the cells that constitute the adult body, such as muscle cells, glandular cells (as liver cells, kidney cells, etc.) ”

“ Each ontogeny (individual) begins in a minute germ cell, in which a small quantity of idioplasm is contained. In the cell divisions, by which the organism grows, the idioplasm divides into as many parts as there are single cells, while it continually increases in quantity in a corresponding degree. The ontogenetic increase of the idioplasm takes place by length growth of the strands—that is, by intercalation of micellæ in each row of cells of the strands, which thereby grow in length without changing the configuration of the cross section. ”

“ One important consequence for morphology results from von Baer's laws of differentiation within the type. If the embryo develops from the general to the special, then the state in which each organ or organ-system first appears must represent the general or typical state of that organ within the group. Embryology will therefore be of great assistance to comparative anatomy, whose chief aim it is to discover the generalised type, the common plan of structure, upon which the animals of each big group are built. ”

“ In the lowest forms of cell differentiation the cells of successive generations are all independent; the ontogenetic period consists of a cycle of generations of unicellular plants. Later the cell generations of an ontogeny are united by parts into plant individuals; the ontogenetic period consists of a cycle of multicellular and unicellular, or only of multicellular plant generations. If all the cell generations of an ontogenetic period have been united into a single individual, the successive plant generations are alike and alternation of generations has ceased. ”

“ Both causes acting together—the phylogenetic configuration of the idioplasm and the successive morphological stages of development of the individual conditioned on it—necessarily result in the ontogeny being the repetition of the phylogeny.
If the whole remaining line of idioplasmic determinants in an ontogeny has reached development, the development of the germ-forming determinants finally follows as well from the configuration [ 16 ] of the idioplasm as from the nature of the organism. The individual is capable of reproduction and the new ontogenies begin in the reproductive cells. ”

“ But we must, to be accurate, distinguish between the plasson of the cytodes and the protoplasm of the cells. This distinction is of the utmost importance for the purposes of evolution. As I have often said, we must recognise two different stages of development in these "elementary organisms," or plastids ("builders") , that represent the ultimate units of organic individuality. The earlier and lower stage are the unnucleated cytodes, the body of which consists of only one kind of albuminous matter—the homogeneous plasson or "formative matter." ”

“ Their significance in the processes of individual development may in truth be far greater than is generally recognised. When in the course of the phylogeny they have played their part as intermediary organs (Vermittelungsorgane) they assume the same function in the ontogeny. Through the stimulus or by the aid of these organs, now become rudimentary, the permanent parts of the embryo appear and are guided in their development; when these have attained a certain degree of independence, the intermediary organ, having played its part, may be placed upon the retired list. ”

“ And of like natures are the criticisms which must be passed on the generalization, that cells are the morphological units of organisms. To continue the simile, the truth turns out to be, that the primitive clay or protoplasm out of which organisms are built, may be moulded either directly, or with various degrees of indirectness, into organic structures. The physiological units which we are obliged to assume as the components of this protoplasm, must, as we have seen, be the possessors of those proclivities which result in the structural arrangements of the organism. ”

“ It does not represent anything actually occurring in nature, except in the case of an embryo produced by some animal which already shows all the tissues which its embryo is destined to reproduce. Thus it establishes no probability of the evolution of complex tissues from simple cells, and leaves altogether unexplained that wonderful process by which the embryo-cell not only divides into many cells, but becomes developed into all the variety of dissimilar tissues evolved from the homogeneous egg ”

“ For all questions of evolution the presence of vestigial organs in the embryo is the important consideration, for they give an indication of near ancestry; the early formation of the embryo concerns a much more remote ancestral period, all vestigial organs of which may well have been lost and obscured by cœnogenetic changes. Let us, then, consider the two things—the vestigial organs and the early formation of the embryo—separately, and see how far my opponents are justified in their statement that my theory contravenes the principles of embryology. ”