II CLEAVAGE 59 



cells intestinal epithelium, gastric epithelium, liver epi- 

 thelium, ectoderm, epithelium of gills, ganglion cells give 

 a mean ratio of 1 : 10-5. There is therefore apparently an 

 increase. 



The mean volume of all the nuclei does not, however, 

 increase to its original size after each division, but diminishes 

 at first to reach its original value once more by the 70-cell 

 stage. The total amount of nuclear material has by this time 

 therefore increased. In the Ascidian Cynthia there is similarly 

 an eventual increase in the total volume of nuclei in spite of 

 the diminution of the mean volume. These results are in 

 exact agreement with those obtained by Fraulein Erdmann 

 for the Sea-urchin. 



4. The fourth question is whether the nuclear and cell- 

 division of cleavage are themselves processes of differen- 

 tiation. 



According to a well-known theory, which is or rather was 

 associated with the names of Roux and Weismann, while the 

 cytoplasm of the ovum was regarded as isotropic or equivalent 

 in all its parts, the internal causes of differentiation were placed 

 in the nucleus, that is, the determinants or different materials 

 on which the appearance in the offspring of the inheritable 

 characters ultimately depend, were imagined to reside in the 

 chromosomes of the nucleus, but to be gradually separated 

 from one another in successive divisions. Nuclear division 

 in other words was qualitative, and through its agency the 

 qualitatively different determinants were distributed to the 

 different cells, there to call forth in the cytoplasm the histo- 

 logical characters to which each was appropriated, the charac- 

 ters which are to the observer the actual sign of differentiation, 

 which is therefore actually produced by cell and nuclear 

 division, and would not occur without it. Further, each cell 

 having received in this way certain determinants and those 

 only, can alone give rise to certain structures, and the causes 

 for the production by it of those structures lie wholly within 

 itself, that is, in its nucleus. Development is therefore a 

 process of self-differentiation of each part of a mosaic-work. 



