318 EDWIN G. CONKLIN 



Before maturation, yolk and cytoplasm intermingle in all 

 parts of the egg and, although the cytoplasm is somewhat more 

 abundant at the animal pole than elsewhere, these two substances 

 are not sharply segregated. When such an egg is strongly cen- 

 trifuged the yolk is driven to the centrifugal pole, while the cyto- 

 plasm is displaced toward the opposite pole, so that these sub- 

 stances come to be partially segregated (figs. 1 and 2), though this 

 segregation is never so complete as it is in later stages. The 

 germinal vesicle goes with most of the cytoplasm to the centrip- 

 etal pole. The entire egg is frequently flattened in the axis of 

 centrifuging, but the germinal vesicle is elongated in that axis, 

 probably owing to the fact that the nucleus is not subjected to 

 external pressure, whereas the egg is. Before the prophase of di- 

 vision, the chromatin is not moved within the germinal vesicle 

 by centrifuging; after the prophase has begun, the chromatin ap- 

 pears to be free to move. This corresponds more or less closely 

 with the conditions found by Kite ('13) in his micro-dissections 

 where the nucleus during resting stages was found to be a gel 

 which becomes more fluid during division phases. In the pro- 

 phases shown in figures 1 and 2 the chromosomes are evidently 

 heavier than other constituents of the nucleus since they collect 

 in the centrifugal end of the germinal vesicle while the nucleolus 

 goes to the centripetal end of the vesicle, thus showing that in 

 this egg the nucleolus is lighter than other nuclear constituents; 

 in other cases it is heavier, as for example in the ovarian egg of 

 the lobster (Herrick, '95) and in the electric-motor nerve cells 

 of torpedo (Dahlgren, '15). The axis of the spindle in figure 2 

 is approximately at right angles to the axis of centrifuging, but 

 as the initial position of the spindle bears no constant relation 

 to the axis of the egg this fact has no particular significance. 



When the membrane of the germinal vesicle of a normal egg 

 dissolves, the first maturation spindle is left in the egg usually 

 at some distance from the surface and at the same time the sper- 

 matozoon enters the egg usually near the vegetal pole. When 

 eggs are strongly centrifuged at this stage there is a fairly sharp 

 separation of cytoplasm and yolk ; the maturation spindle is car- 

 ried along with the cytoplasm thus showing that it is not at this 



