122 



Soon after the work was begun it became clear that the nuclei 

 in some regions were dividing mainly by mitosis and in other regions 

 mainly by amitosis. Furthermore, it was noticed that a region in 

 which amitosis predominated, may, upon further differentiation, become 

 one in which mitosis predominates, and vice versa. 



The data secured from a study of the germ layers will serve to 

 show in a general way this regional occurrence of amitosis and mitosis. 

 By such a study it was found that shortly after gastrulation 2.66 per 

 cent of the ectoderm cells were dividing mitotically and 1.08 per cent 

 amitotically, while in the entoderm there were only 0.81 per cent of 

 the cells in mitosis as against 5.78 per cent in amitosis (ser. 256). 

 Four hours after laying, when the primitive streak can first be detected 

 in sections, there were 3.02 per cent of the ectoderm, 4.49 per cent 

 of the entoderm, and 2.30 per cent of the mesoderm cells dividing by 

 mitosis, while dividing amitotically were 0.53 per cent of the ectoderm, 

 1.79 per cent of the entoderm, and 5.49 per cent of the mesoderm 

 cells. Following this period there is very little change in the relative 

 proportions (sers. 328, 189). 



These data are summarized in table IL 



Table II. 



1) The term mesoderm is here used to designate the primitive streak when it 

 can first be seen in sections. 



Discussion and Conclusion. 

 A study of the regional occurrence of mitosis and amitosis re- 

 veals the fact that the former is found mainly in slowly and the latter 

 in rapidly growing parts of the blastoderm ^). Thus, just after gastru- 

 lation when the entoderm is increasing very rapidly in its forward 

 growth through the subgerminal cavity, 5.78 per cent of the entoderm 

 cells are dividing amitotically and only 0.81 per cent mitotically (ser. 256). 



1) Dr. Child ('07) finds a similar condition in Moniezia. 



