100 



BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 



the central clump has not yet broken up into yolk bodies. Germina- 

 tive vesicle stellate. 



4. Central yolk clump has broken up into yolk bodies. Germinative 

 vesicle stellate. 



Out of 48 eggs measured, the various sizes, expressed in micra (ya), 

 were distributed as follows : — 



From this table, it is seen that the classes established on morphologi- 

 cal grounds contain groups of similar ova, and hence represent the actual 

 sequence of events. The column of averages especially shows well 

 how the germinative vesicle diminishes with the growth of the ovum. 



Among the fifty-two ova examined, four exceptions were encountered 

 that would not fit well into any of the classes. Two of these were 

 small ova, about on the transition line between the second and third 

 classes, but in which the cytoplasmic conditions w^ere very indistinct. 

 The other two belong to an entirely different category, and from their 

 rare occurrence in my early series, from which the above data were 

 taken, as well as in the later series, they must be considered abnormal. 

 In them the yolk bodies were completely differentiated in all regions, 

 but the contents of the germinative vesicle was finely granular and not 

 separated by a membrane from the yolk. 



From this account it follows that the shrinking of the germinative 

 vesicle is a continuous process, and much simpler than Davidofif con- 

 sidered it. In attempting to interpret his results in the light of these 

 investigations, there is but one point that offers any obstacle. That is 

 his initial stage in the shrinking, in which the membrane of the 

 vesicle disappears and its contents penetrate between the neighboring 



