The writer has observed more closely the trans­forma­tion of an organ into more undifferentiated material in Campanularia (Fig. 33), a hydroid.[163] This organism shows a remarkable stereotropism. Its stolons attach themselves to solid bodies, and the stems appear on the side of the stolon exactly opposite the point or area of contact with the solid body. The stems grow, moreover, exactly at right angles to the solid surface element to which the stolon is attached. If such a stem be cut and put into a watch glass with sea water, it can be observed that those polyps which do not fall off go through a series of changes which make it appear as if the differentiated material of the polyp were trans­formed into undifferentiated material. The tentacles are first put together like the hairs of a camel’s-hair brush (Fig. 34), and gradually the whole fuses to a more or less shapeless mass which flows back into the periderm (Fig. 35). It follows from this that in this process certain solid constituents of the polyp, e. g., the cell walls, must be liquefied. This undifferentiated material formed from the polyp may afterward flow out again, giving rise to a stolon or a polyp; to the former where it comes in contact with a solid body, to the latter where it is surrounded by sea water. These observa­tions suggest the idea of reversibility of the process of differentia­tion of organs and tissues, in certain forms at least. We have to imagine that some of the cells or interstitial tissue is digested and that as a consequence the organ loses its characteristic shape.

Giard and Caullery have found that a regressive metamorphosis occurs in Synascidians, and that the animals hibernate in this condi­tion. The muscles of the gills of these animals are decomposed into their individual cells. The result is the forma­tion of a parenchyma which consists of single cells and of cell aggregates resembling a morula.[164]

Driesch,[165] experi­menting on the regenera­tion of an Ascidian, found that when he cut off the gills and siphons of the animal the portion removed was able to regenerate a whole animal. The gill-piece excised contained no heart, no intestine, and no stolon, and all these organs were regenerated from the gills. In a number of cases the regenera­tion took place by bud forma­tion at the edge of the wound, but in other cases the gills were trans­formed into an undifferentiated mass of tissue from which the missing parts of the animals arose by budding and new gills were formed.

It is probable that the two cases are only quantitatively different. In both, autodiges­tion of certain cell constituents and possibly of whole cells must take place in order to obtain material for the forma­tion of the lost part of the Ascidian. If an interstitial tissue is digested it becomes a ques­tion of how much of this tissue undergoes hydrolysis. If there is little destroyed the old shape of the gills remains, if too much is digested the old gills become a shapeless mass in which a certain number of the old cells are maintained and give rise to the new animal by cell division. The material for the new organs must of course be furnished from old cells which have been digested.

If regenera­tion takes place in pieces which take up no food the newly formed organs must originate from material absorbed from cells of the animal which are hydrolyzed and whose material serves as food for those cells which grow. Very often this process of diges­tion takes place without loss of the total form of the organ and is overlooked by the pure morphologists. In Campanularia also the process of collapse described above is only apparent in a fraction of the cases as in Driesch’s observa­tions on Clavellina.[166] It is also possible that the red and yellow entoderm cells which gather at the end where the new polyp forms furnish the material which is utilized for the process of growth of the cells from which the tentacles arise (with or without giving off specific “hormones” besides).

6. We have mentioned the ideas concerning a design, or “entelechy,” acting as a guide to the developing egg and have shown that this revival of Platonic and Aristotelian philosophy in biology was due to a misconcep­tion; namely, that the egg consisted of homogeneous material which was to be differentiated into an organism. For this supernatural task supernatural agencies seemed required. But we have seen that the unfertilized egg is already differentiated in a way which makes the further differentia­tion a natural affair. This idea of a quasi superhuman intelligence presiding over the forces of the living is met with in the field of regenera­tion, and here again it is based upon a misconcep­tion. The lens of the eye is formed in the embryo from the epithelium lying above the so-called optic cup (the primitive retina). Where this retina touches the epithelium the latter begins to grow into the cup, the ingrowing piece of epithelium is cut off and forms the lens, which probably under the influence of substances secreted by the optic cup becomes transparent. Certain animals like the salamander are able to form a new lens when the old one has been removed by opera­tion, but the new lens is formed in an entirely different way; namely, from the upper edge of the iris. G. Wolf, who observed this regenera­tion used it to endow the organism with a knowledge of its needs; the idea of a Platonic preconceived plan or an Aristotelian purpose suggested itself. But it can be shown that the organism does in this case what it is compelled to do by its physical and chemical structure.

Uhlenhuth[167] has shown by way of tissue culture that the cells of the iris cannot grow and divide as long as they are full of pigment granules as they normally are. When the fine superficial membrane of the iris is torn the pigment granules fall out and the cells can now grow and multiply. If the lens is taken out of the eye of the salamander the fine membrane of the iris is torn and the pigment cells at the edge (especially the upper edge) lose their pigment granules which fall down on account of their specific gravity. As soon as this happens the cells will proliferate. A spherical mass of cells is formed which become transparent and which will cease to grow as soon as they reach a certain size. The unanswered ques­tion is: Why does the mass of cells become transparent so that it can serve as a lens? The answer is that young cells when put into the optic cup always become transparent no matter what their origin; it looks as if this were due to a chemical influence exercised by the optic cup or by the liquid it contains. Lewis has shown that when the optic cup is transplanted into any other place under the epithelium of a larva of a frog the epithelium will always grow into the cup where the latter comes in contact with the epithelium; and that the ingrowing part will always become transparent. This leaves us then with one puzzle still: Why is the growth of the lens limited? The limita­tion in the growth of organs is one of the most important problems in growth and organ forma­tion, though unfortunately our knowledge of this topic is inadequate.

7. The botanist J. Sachs was the first to definitely state that in each species the ultimate size of a cell is a constant, and that two individuals of the same species but of different size differ in regard to the number, but not in regard to the size of their cells.[168] Amelung, a pupil of Sachs, determined the correctness of Sachs’s theory by actual counts. Sachs, in addi­tion, recognized that wherever there were large masses of protoplasm, e. g., in siphoneæ and other cœloblasts, many nuclei were scattered throughout the protoplasm. He inferred from this that “each nucleus is only able to gather around itself and control a limited mass of protoplasm.”[169] He points out that in the case of the animal egg the reserve material—fat granules, proteins, and carbohydrates—are partly trans­formed into the chromatin substances of the nuclei, and that the cell division of the egg results in the cells reaching a final size in which each nucleus has gathered around itself that mass of protoplasm which it is able to control. Morgan[170] and Driesch[171] tested and confirmed the idea of Sachs for the eggs of Echinoderms. We stated in the previous chapter that Driesch produced artificially larvæ of sea urchins of one-eighth, one-fourth, and one-half their normal size by isolating a single cleavage cell in one of the first stages of segmenta­tion of the fertilized sea-urchin egg. He counted in each of the dwarf gastrulæ resulting from these partial eggs the number of mesenchyme cells and found that the larvæ from a one-half blastomere possessed only one-half, those from a one-fourth blastomere only one-fourth, and those from a one-eighth blastomere only one-eighth of the number of cells which a normal larva developing from a whole egg possessed. Moreover, he could show that when two eggs were caused to fuse so as to produce a single larva of double size, the gastrulæ of such larvæ had twice the number of mesenchyme cells. Driesch drew the conclusion from his observa­tions that each morphogenetic process in an egg reaches its natural end when the cells formed in the process have reached their final size.

Since each daughter nucleus of a dividing blastomere has the same number of chromo­somes as the original nucleus of the egg, it is clear that in a normally fertilized egg each nucleus has twice the mass of chromo­somes that is contained in the nucleus of a merogonic egg, i. e., an enucleated fragment of protoplasm into which a spermato­zoön has entered and which is able to develop. Such a fragment has only the sperm nucleus. This phenomenon of merogony was discovered by Boveri and was elaborated by Delage.[172] Boveri, in comparing the final size of the cells in normal and merogonic eggs after the cell divisions had come to a standstill, found that this size is always in propor­tion to the original mass of the chromatin contained in the egg; the cells of the merogonic embryo, e. g., the mesenchyme cells, are only half the size of the same cells in the normally fertilized embryo. Driesch furnished a further proof of Boveri’s law, that the final ratio of the mass of the chromatin substance in a nucleus to the mass of protoplasm is a constant in a given species. Driesch compared the size of the mesenchyme cells in a sea-urchin embryo produced by artificial parthenogenesis with those of a normally fertilized egg and found them half of the size of the latter. When the fertilized eggs and the partheno­genetic eggs are equal in size from the start,—which is practically the case if eggs of the same female are used,—the process of the forma­tion of mesenchyme cells comes to a standstill when their number in the normally fertilized eggs is half as large as the final number in the partheno­genetic egg.[173] Boveri’s results as well as those of Driesch were obtained by counting the cells formed by eggs of equal size and not by simply measuring the size of the cells. It is most remarkable that certain apparent excep­tions to Boveri’s law which Driesch has actually found had been predicted by Boveri.