121 



been included, while in mitosis, all stages from the earliest prophases 

 to the late teleophases have been counted. 



Table I. 



1) The "age" refers to the number of hours after fertilization. Since laying 

 occurs 41 hours after fertilization, it is easy to determine whether any one of the 

 above series was taken before or after laying — e. g., series 304 was taken 10 hours 

 before and series 328 four hours after laying. 



My preparations do not cover the first stages of development, 

 and consequently the earliest egg studied is in a mid-cleavage stage 

 (ser. 194). In this series there is a preponderance of mitoses over 

 amitoses, a condition found to be true in all the eggs studied at this 

 period of development. In passing to successively later cleavage stages, 

 however, one finds that this difi'erence in frequency grows less and 

 less (sers. 304, 394) until the gastrulation period is reached, when 

 mitoses and amitoses occur in about equal numbers (ser. 284). 

 Immediately after the closing of the blastopore (ser, 256) the number 

 of direct divisions slightly exceeds that of indirect — this ratio 

 existing until after the time of laying. During the earliest stages of 

 the primitive streak (ser. 328) mitoses are again more abundant than 

 amitoses and continue to increase until the lateral wings of mesoderm 

 are well formed (ser. 189). 



Judging from the above data it seems very probable that the 

 nuclear divisions of the developing blastoderm are at first wholly 

 mitotic, but later, amitosis sets in and gradually increases, in pro- 

 portion to mitosis, until a maximum is reached, after which it 

 decreases. 



The Regional Occurrence of Amitosis and Mitosis. 

 The above data were introduced to show in a general way the 

 frequency of amitosis during early development, but if no further facts 

 were presented the reader would have a very imperfect knowledge of 

 amitosis as it occurs in this form, and little or no idea of its signi- 

 ficance. 



