240 RE GENERA TION 



eccentric position. The embryos resemble in every respect the incom- 

 plete embryos from isolated blastomeres. It is important to note that 

 although the embryos from isolated blastomeres resemble those from 

 pieces of the segmented egg, in the former case the nucleus has 

 divided once, and each blastomere contains half of the original 

 nucleus, while in the latter case the entire segmentation nucleus is 

 present in the piece. These facts seem to show that in this egg the 

 incomplete development is directly connected with the protoplasm, 

 and not with the nucleus, a view that is maintained by Driesch and 

 Morgan in connection with these experiments. 



It was found in one or two instances that the nucleated pieces 

 divided in the same way that the whole egg did, except that the blas- 

 tomeres are proportionately smaller. From pieces of this kind whole 

 embryos of small size developed. In this case we must suppose that 

 the protoplasm has succeeded in rearranging itself into a new whole 

 of smaller proportions. 1 



Crampton ('96) has shown in a mollusk, Ilyanassa obsoleta, that 

 when a blastomere is separated from the rest, the cleavage proceeds 

 as though the blastomere or its products were still present, and the 

 larva is defective in those organs that are normally derived from that 

 blastomere. These results are in line with those on the ctenophore 

 egg. Fischel (1900) has also made some experiments on the seg- 

 mented egg of the ctenophore, and has confirmed several of the results 

 obtained by Driesch and Morgan. In addition he has tried the effect 

 of disturbing the first-formed cells by pushing them over each other, 

 so that their relative positions are changed. He finds as a result 

 that the paddles, sense organ, etc., appear in unusual positions, and 

 the latter may be doubled. This shows that we must regard the 

 material or structural basis of the organs as present very early in 

 the different parts of the egg, and that the organs develop without 

 much regard to their relation to other organs. 



Ziegler ('98) has also made some observations on the egg of this 

 same ctenophore, that bear directly on some of the questions here 

 raised. His study of the cleavage shows that the micromeres arise 

 from the part of the egg that is opposite the pole at which the 

 first cleavage furrow appears the animal pole. Fischel's results 

 have shown that the paddles and the sense organs arise from these 

 micromeres, for, if the latter are displaced the former are also. 



Ziegler performed the experiment of cutting off that part of an egg 

 (which has just begun to divide) lying opposite the region in which 

 the first furrow has appeared. In this way there was removed 

 from the unsegmented egg the part from which the micromeres 



1 We offered as a possible explanation in this case that the egg had been cut in two 

 symmetrically with reference to the eccentric nucleus. 



