246 REGENERATION 



present. Ziegler's results show that, even after the removal of that 

 part of the egg from which the micromeres develop, the segmenta- 

 tion may still be like that of the whole egg, and this shows that the 

 egg has great powers of recuperation (at least in a symmetrical 

 plane), so far as its protoplasm is concerned. If, however, it is true 

 that when a part is cut off unsymmetrically the protoplasm cannot 

 reorganize itself, then the conclusion that Driesch and Morgan drew in 

 regard to the protoplasm will hold, provided, as seems to be the case, 

 the smaller blastomere of the first two is large enough to produce the 

 typical structures. The main point is this : If the protoplasm re- 

 adjusts itself after the operation, so that the piece divides as a whole, 

 a complete embryo develops ; if, however, the protoplasm does not 

 readjust itself, and the piece divides as a part, an incomplete em- 

 bryo is formed. Since in both cases the same nucleus is present, 

 and since the difference is obviously connected with a change in the 

 protoplasm, it seems much more probable that the phenomenon of 

 whole and half development is connected with the protoplasm and 

 not with the nucleus. 



The hypothesis that Pfliiger, Hertwig, and Driesch have adopted, 

 namely, that the cleavage divides the egg into potentially equal parts, 

 stands in sharp contrast to the Roux-Weismann conception of devel- 

 opment. There are two ideas in the former view which should be 

 kept, I think, clearly apart : the first is, that the blastomeres are 

 potentially equal (isotropous), because they are exactly alike ; the 

 second is, that despite the differences that may exist amongst them 

 they are still potentially able to do the same thing, i.e. they are toti- 

 potent. The former alternative is that adopted by Pfliiger, Hertwig, 

 and Driesch ; the latter view, to which Driesch seems more inclined 

 in his later writings, is the one that I should prefer. 1 The first 

 four blastomeres of the sea-urchin's egg appear to be exactly alike, 

 and we find that each can make a whole 'embryo. If we assume, 

 however, that despite their likeness and their totipotence they are 

 different in so far as there is present in the protoplasm a bilateral 

 structure, we are nearer, in my opinion, to the truth ; for, unless we 

 assume the bilateral structure to be determined later by some exter- 

 nal factor, of which there is no evidence, we must suppose that after 

 fertilization, at least, there must be a bilateral structure to the proto- 

 plasm, and this view is borne out in one sense by the subsequent 

 mode of cleavage of the blastomeres if they are separated. Whether 

 this bilaterality of the fertilized egg leads to the bilaterality of the 

 cleavage is, however, a different question. In some cases this 

 appears to be the case, in others it is clearly not the case, and we 

 must suppose that some other condition determines the bilaterality 



1 As stated in my article on " The Problem of Development," 1900. 



