A STATISTICAL STUDY OF MITOSIS AND AMITOSIS. 



227 



size ; contain no granules that stain with orange G and are 

 occasionally almost completely rilled with a vacuole, so that in 

 certain localities I feel reasonably certain that two adjoining 

 vacuoles often represent two cells. The nuclei of the entoderm 

 in this region are small in comparison with those from other 

 places. 



The Nuclear Phenomena in the Entoderm. 

 The fact that amitosis occurs in the entoderm of Fasciolaria 

 embryos, was so far as I know first definitely asserted by Osborn. 

 "The entoderm," says Osborn ('04 I.), "is composed of cubical 

 cells in which one finds all stages of direct division." 1 Fig. 6 

 represents some of these divi- 

 sions. The nuclei shown in 

 this picture were enlarged from 

 the same sections from which 

 Fig. 4 was compounded. A 

 and b are removed from their 

 cells. In one of them a the 

 finely divided chromatin gran- 

 ules exhibit a slightly reticu- 

 lar arrangement and consider- 



able condensation along the 

 inner surface of the nuclear 

 membrane. Here and there are 

 larger dense collections of 

 these granules suggesting an interrupted skein. The nucleus in 

 question is markedly bilobed, the larger lobe having a small nucle- 

 olus, the smaller lobe a large nucleolus. Separating the two 

 lobes incompletely is a very delicate interrupted membrane, which 

 on close inspection was found to be composed of a dense col- 

 lection of granules like those lining the inside of the nuclear mem- 

 brane. I have seen these granular boundaries so frequently be- 

 tween the lobes of what I take to be dividing nuclei, that I con- 

 clude that cleavage is in many cases initiated by a granular plate 

 that grows inward from the nuclear wall. Nucleus b is very much 



1 These direct divisions were interpreted by Osborn in a later paper ('04, II.) as 

 growth phenomena, a view supportable, as the sequel will show, by much additional 

 evidence. 



Fig. 6. Cells and nuclei from the ex- 

 cretory zone of the entoderm. 



