166 bulletin: museum of comparative zoology. 



cases the two processes take place at different times, so that it seems 

 probable that chromatin and nucleolus have no fixed relation to each 

 other. 



The time of disappearance of the nucleolus is not constant. For 

 example, Figures 1 and 2 are from eggs of the same stage, having been 

 killed ten hours before the usual time of liberation of the medusa; 

 yet in one case the nucleolus is large, apparently having undergone 

 little diminution of size, and in the other it has almost disappeared. 

 Figures 3 and 4 also represent eggs of the same age ; here the differences 

 in the size of the nucleoli are still more marked. A comparison of 

 these two sets of eggs also makes clear the lack of agreement between 

 the disappearance of nucleoli and the appearance of chromatin strands. 

 While the eggs are apparently of similar age, the chromatin appears to 

 be more concentrated in those cases (Figs. 1, 3) where the nucleolus 

 is larger, more diffuse where the nucleolus is smaller (Figs. 2, 4). I 

 have no stages intermediate between these four and those in which 

 the nucleolus has already disappeared. Figures 5-7 show that when 

 the germinative vesicle has advanced further toward the formation of 

 polar cells no nucleoli are present. It seems clear from Figures 1-7 

 that the nucleolus disappears gradually, perhaps by dissolving rather 

 than by breaking up into fragments. 



With regard to the nucleolus, then, the evidence seems clearly to 

 lead to these conclusions: (1) it is mainly, if not entirely, a non- 

 chromatic body at all stages from the young oocyte to the mature egg; 

 (2) it entirely disappears, probably by dissolving, before the nuclear 

 membrane has been ruptured; (3) the time of its disappearance 

 varies in different eggs; (4) there is a direct connection between the 

 nucleolus and the linin network, so that an exchange of material may 

 take place between nucleolus and chromatin along the linin network, 

 but not in the form of prepared chromatin. 



2. Formation of Polar Cells. Before describing the details of 

 polar-cell formation something should be said regarding the time of its 

 occurrence. In examining living eggs just after they had been dis- 

 oharged from the medusa (this was done by compressing the eggs 

 slightly under a cover glass), the nuclei appeared small and without 

 nucleoli. Evidently the polar cells had already been formed, though 

 they w^ere no longer discoverable, doubtless because they had been 

 lost in the water. Eggs similarly examined a few hours before the 

 discharge of the medusae show the germinative vesicle, which usually 

 contains a nucleolus. The conclusions reached from the study of 

 fresh eggs were confirmed by the examination of sections of fixed eggs. 



