Regeneration

by Thomas Hunt Morgan

Roux also tried to show that if, after the second cleavage has been completed, the two blastomeres that lie on opposite sides of the first cleavage plane are killed by a hot needle, the remaining two produce either an anterior or a posterior half of an embryo. An embryo derived from the two “anterior” blastomeres is represented in [Fig. 61], D. The anterior half of the body is present. Posteriorly the half-embryo abuts against the injured half. It is possible, I think, that this embryo may represent the anterior half of a whole embryo of half size that has been prevented from closing in posteriorly by the mass of injured material of the undeveloped blastomere. Roux did not determine positively whether the two “posterior” blastomeres could give rise to posterior half-embryos; one embryo in his opinion appeared to bear out this interpretation. This part of Roux’s work is, it seems to me, not so satisfactory as the part dealing with the first two blastomeres, and we may leave it, for the present, out of the discussion, and consider only the result of the first experiment, in which one of the first two blastomeres was injured. Since the problems involved in the two cases are essentially the same, nothing will be lost by dealing with the first case alone.

The uninjured blastomere first gives rise to a half-embryo. After this has been accomplished, other changes take place that “reorganize,” according to Roux, the material of the injured half in such a way that the missing half of the embryo is formed by a process that Roux calls “post-generation.” This process can be studied only by means of sectioning the embryos, and since the eggs may be injured to a varying extent, there must be some uncertainty in making out the sequence of events. It is found that the yolk of the injured blastomere is vacuolated in places, and that the protoplasm in the path of the needle has been killed ([Fig. 61], A). Irregular pieces of chromatin are found in the protoplasm, which seem to come from an irregular breaking up of the nucleus.

The changes that lead to the reorganization of the injured half may take place at different times in different eggs. Roux describes three kinds of reorganization phenomena. The first includes the formation of new cells in the injured half. Nuclei, surrounded by finely granular protoplasm, appear in the protoplasm of the injured blastomere. These nuclei arise from two sources: in part from the scattered chromatin of the injured blastomere itself, and in part from nuclei, or from cells without walls that have emigrated from the developing half. Around these nuclei, as centres, the protoplasm (with its contained yolk) of the injured half breaks up into cells. This cellulation of the yolk may take place in different eggs at different times. In some cases it may not have appeared as late as the gastrula stage of the uninjured half; in others, it may take place at the time when the uninjured half is segmenting.[110] The formation of the cells in the injured half begins always near the developing half, and extends thence into the injured parts. The new cells are of different sizes, but are larger than those of the uninjured half.

The cellulation of the yolk takes place only in the least injured parts of the protoplasm. Where the protoplasm and yolk have been much injured, they are changed over by the second method of reorganization. This part of the blastomere is either actually devoured by wandering cells, or is slowly changed under the influence of the neighboring cells, so that it becomes a part of these cells.

The surface of the injured half is covered over by ectoderm that grows directly from the developing half (third method of reorganization),—at least this happens where the protoplasm has been much injured. In other parts of the injured half the new cells that have appeared in this part, and that lie at the surface, become new ectoderm.

Post-generation now begins in the reorganized and cellulated half; the cells become changed over into the different layers and organs that make the new half-embryo. A few hours or a night is sometimes sufficient to change a hemi-embryo into a whole embryo. The new half-medullary fold develops from the new ectoderm to supplement the half already present. The mesoblast appears over the side. Its upper part seems to come from the uninjured mesoderm that has grown over to the other side, but this is added to at the free edge by cells that belong to the newly cellulated part. The new differentiation is, in general, in a dorso-ventral direction. The lacking half of the archenteron arises in connection with the half of the archenteron already present in the hemi-embryo. The yolk cells arrange themselves radially, and a split appears in the post-generated part, extending from the archenteron of the hemi-embryo. The split opens, and the new half-archenteron appears. In general, Roux states, the post-generation of the organs of the injured half proceeds from the already differentiated germ-layers of the hemi-embryo. The post-generation begins where the exposed surfaces of the germ-layers of the hemi-embryo touch the newly cellulated regions of the injured half.

It is most difficult to account for these post-generative changes, since the new part has, according to Roux, a double and even a three-fold origin. The pieces of the old nucleus, he admits, may take a part in the formation of the new cells; wandering cells migrate from the yolk mass of the old half into the new, and the cells of the formed germ-layers may be pushed over to the other side. Since a certain share, and perhaps a large share, of the new cells comes from the hemi-embryo, it is clear that, in addition to the power of self-differentiation shown by the uninjured blastomere, we must also ascribe to it certain regenerative powers, at least to the extent that each kind of cell that comes from it can give rise in the injured half to cells like itself, and produce similar structures in the other half.

If then, as Roux supposes, the development of the egg consists in an orderly, qualitative series of changes that lead to the subsequent differentiation, we must also suppose that the new parts are gifted with latent powers by virtue of which they can re-create all parts of the other half. Roux supposes, in fact, that each cell carries with it a sort of reserve-plasm, that is dormant in ordinary development but is awakened when any disturbance of the normal development takes place. Objections have been made to this subsidiary hypothesis, since the addition of this to the original assumption of a series of qualitative changes involves such complications that the view can hardly be considered a probable one. This objection is, I think, not as strong as certain critics believe, since the facts of development show beyond a doubt that although the egg has the power of progressive change it has also, as certain experiments show, the power of reorganization, if the ordinary course of events is interrupted. This admission by no means throws us back upon Roux’s hypothesis, for, as will be shown later, a different conception of the development may better account for both phenomena.

Inasmuch as a good deal of discussion has taken place in regard to the process of post-generation described by Roux, it should be stated that Endres and Walter reëxamined the process, and found, as had Roux, that the reorganizing cells migrate from the uninjured to the injured side, and around them the protoplasm of that side makes new cells. They found that the injured half is directly overgrown by the ectoderm from the developing half. When the material of the injured blastomere is only incompletely reorganized, there is formed, after post-generation, an embryo that has a protrusion of yolk in the dorsal part of the body. When the injured material is completely worked over, a perfectly formed embryo may result. The typical half-embryos that Roux obtained were also obtained by Endres and Walter. They deny that whole embryos develop from one of the first two blastomeres, as Hertwig affirms.