In some cases an extra digit or toe may regenerate on the leg of a salamander, and this too has been interpreted as a return to an ancestral condition. But Tornier has shown, as has been stated, that several additional digits, or even a whole extra hand, may be produced by wounding the leg in certain ways, and these too would have to be interpreted as ancestral, if the hypothesis is carried out logically. It has been shown by King that one or more additional arms may be produced in a starfish by splitting between the arms already present, and if we accepted evidence of this sort as having any value in interpreting lines of descent we should conclude[106] that the ancestors of the starfish had six, seven, or more arms according to the number that can be produced artificially, etc. Therefore, until further evidence of a more convincing kind is forthcoming, we can safely, I think, decline to accept the results, so far known, as having any value in interpreting the relationships or the descent of the animals.

CHAPTER XI
REGENERATION IN EGG AND EMBRYO

Not only do adult organisms have the power of regeneration, but embryos and larval forms possess the same power, and even portions of the segmenting, and also the unsegmented, egg may be able not only to continue their development, but in many cases to produce whole organisms. Haeckel observed in 1869-1870 that pieces of the ciliated larvæ of certain medusæ, and even pieces of the segmented egg, could produce whole organisms. The more recent experiments of Pflüger (’83) and of Roux (’83) on the frog’s egg mark, however, the beginning of a new epoch in embryological study. The explanation of this is to be found, I think, not only in the introduction of experimental methods, but also in the fact that Pflüger and Roux realized the important theoretical questions involved in their results.

Pflüger’s experiments were made by changing the conditions under which the egg develops in order to determine what factors control the development. Since these experiments were made with whole eggs, the problems of regeneration were not directly involved in his results, although his conclusions are of great importance in connection with questions concerning the regeneration of the egg. A part of Roux’s work dealt directly with the development of a new organism from a piece of the egg or of the embryo. Roux’s principal discovery[107] (’88) was that a half-embryo develops from either of the first two blastomeres of the frog’s egg, if the other blastomere has been injured or destroyed, but that subsequently the missing half of the embryo is “post-generated.” Roux was led to this experiment by his discovery that the plane of the first cleavage of the egg corresponds very often to the median plane of the body of the embryo.[108] This relation suggested that there might be some causal connection between the two phenomena in the sense that the first cleavage plane divides the material for the right side of the body from that of the left side. In a descriptive sense this would be, of course, true if the two planes do really correspond, and if there was no later shifting of material across the middle line, but whether the two phenomena are causally connected, or are merely due to a coincidence, could only be determined by further experiment. The observations themselves are not beyond question, for the two planes do not always coincide, and may be even ninety degrees apart. These cases of divergence were thought by Roux to be due to an unobserved shifting of the developing embryo, but it is improbable that all cases can be accounted for in this way.

Fig. 61.—After Roux. A. Section of semi-blastula of frog’s egg. B. Half-embryo. C. Cross-section of last (reversed right and left in B and C). D. Anterior half-embryo.

Roux carried out his experiment by plunging a hot needle into one of the first two blastomeres, so that it is injured to such an extent that its development is prevented. The same needle, without heating again, was used for one or two other eggs, for, if the needle had been so hot in the first instance that both blastomeres had been injured by the heat, this might not happen in the second or the third egg. It was found that amongst the eggs that had been operated upon in this way, some had been so much injured that neither blastomere developed, others had been so little injured that both blastomeres developed, but in the successful operations the uninjured blastomere developed, while the injured one did not. In the last case the uninjured blastomere divided, and produced a large number of cells. A segmentation cavity was present in the upper part of the hemisphere ([Fig. 61], A). The injured half remained in contact with the other, completing the sphere, but it did not segment. A half-embryo developed from the uninjured half, as shown in [Fig. 61], B, C. This embryo has a half-medullary fold along the side in contact with the injured half. At the anterior end somewhat more than half a head is present, and at the posterior end there is a half-blastopore. The cross-sections[109] ([Fig. 61], C), through the embryo, show that beneath the half-medullary fold a rod-like notochord is present, which is made up apparently of fewer cells than the normal notochord, but it has, in cross-section, a round and not a half form. At the side, the mesoderm is present, as in the normal embryo, and it has produced the characteristic mesoblastic somites. An archenteron is formed in the half-embryo, and, since it is smaller than the normal, it may, perhaps, be called a half-archenteron. The embryo is, therefore, in most respects a half-structure. The head is, however, nearly a whole head, but whether this is due to a whole head developing out of material derived entirely from one of the two blastomeres, or whether, as Roux supposes, a portion of the material of the injured blastomere has been worked over, i.e. “post-generated,” remains, I think, an open question.

The results of this experiment seem to confirm Roux’s conjecture that the material of each of the first two blastomeres is of such a sort that it gives rise to half the embryo, and, if so, there would be some probability that there is a causal connection between the first cleavage and the separating out of the parts of the embryo. In fact, Roux drew this conclusion, and even attempted to show how such a qualitative division is brought about. It should not be overlooked, however, that this conclusion goes beyond the legitimate bounds of deduction from the results, since the half-development takes place while the injured half retains its connection with the developing half, the former still remaining alive. On the other hand, the presence of the injured half makes the experiment more suitable to demonstrate that each of the first blastomeres gives rise, under normal circumstances, to half of the embryo. If one half had been removed, we can foresee that its absence might lead to other complications that would affect the result.

The most important outcome of this experiment is, I think, to show that a half-structure may develop by itself, i.e. that there is a certain amount of independent power of development in the parts of the egg.