m'clung: cytology and taxonomy. 211 



In the fertilized egg the conditions are much different. Pre- 

 ceding the entrance of the spermatozoon the synaptic chromo- 

 somes have operated in the process of building up both a large 

 nucleus and a large cyfosome. At maturation most of the non- 

 chromatic material is discharged into the cytosome, so that 

 upon their conjugation the pronuclei are probably equivalent 

 in other respects aside from the chromosomes. It has been 

 shown that in the early stages of embryo formation there is an 

 actual contribution of chromosomic material to the cytoplasm. 

 That the chromosomes increase in size after each division by 

 taking up material from the cytoplasm is a common observa- 

 tion. There is thus a constant circulation of material through 

 nucleus and cytosome, and that, I consider, offers an adequate 

 explanation of the means of differentiation, for if the chromo- 

 somes were individually different they would respond adapt- 

 ively under the varying conditions of development. 



Assuming this as an explanation of the means of differentia- 

 tion, how can we conceive the operation of the processes of de- 

 velopment in relation to the germ-cells, which divide at the 

 same time as the body-cells and yet suffer no differentiation? 

 The only observations that would serve as a key to this problem 

 would indicate that the absence of differentiation is due to the 

 retention of the entire chromosome complex unchanged. If 

 differentiation is due to the interchange of material between 

 the chromosomes and cytoplasm under like conditions, it would 

 naturally follow that the absence of differentiation might re- 

 sult were there no such interchange. The few observations 

 that we have would support this view. We are, however, much 

 in need of a thorough knowledge of the changes undergone by 

 all the cell elements during ontogenetic development. We 

 would have a much broader foundation for our theories if we 

 knew the nature of the chromosome complex in the various 

 cells of the body as these became differentiated. It would then 

 be possible to say, with more assurance than our present scanty 

 observations incline us to do, that the germ-cells preserve the 

 chromosome complex unaltered, while it becomes progressively 

 and variously changed during histogenesis. 



It is very clear that we cannot look to any variation in the 

 operation of mitosis as a cause for somatic differentiation, 

 since our observations indicate that the process is designed to 

 produce exact chromosome equivalents. If the chromosomes 

 are the controlling centers of differentiation, then the only con- 



