180 BULLETIN OF THE 



animal rather than the vegetative pole which shows the greater degree of 

 flattening. This modification of the general form of the yolk reaches its 

 maximum as the constriction at the base of the polar cylinder begins to 

 deepen. But it is not alone a flattening which is noticeable at this time : 

 the whole contour of the yolk becomes conspicuously modified. Whereas, 

 at the first appearance of the protuberance, it has already become slightly 

 flattened, it still remains symmetrical as regards the polar axis. Very 

 soon, however, it becomes irregular, and more or less angular in its 

 outline, and often appears remarkably unsymmetrical. During these 

 few minutes it is constantly undergoing a slow change of form, which 

 seems to afl'ect every part of the yolk, and to be accompanied by redis- 

 tributions of the granular substance of the vitellus, so that now one and 

 then another portion becomes more opaque. As the detachment of the 

 polar globule comes nearer to realization, these changes become less no- 

 ticeable,* and finally, when the act is completed, the yolk has resumed 

 its spherical form, and shows the same clear, even outline which had 

 previously characterized it for so long a time. At the close of this act, 

 a single, poorly defined clear spot is seen near the surface at the animal 

 pole. The region of this pole still retains to a considerable extent its 

 transparency, and a thin surface portion of clear protoplasm envelops 

 the yolk on all sides. It is thickest at the animal pole, and thinnest at 

 the vegetative. The portion immediately underlying the polar globule 

 sometimes presents a peculiar striate aspect, which I have been unable 

 fully to explain by other methods of study. The appearance is that of 

 fine parallel stria,tions, sometimes having the same direction as the 

 animal axis (Fig. 49), sometimes oblique to it, or, on the other hand 

 (Fig. 27), of two systems of parallel lines crossing each other at a con- 

 siderable angle. These systems of striations seemed to be changing in 

 position, yet without any recognizable regularity. They are probably 

 astral rays or fibres of a nuclear spindle. (See below.) Gradually the 

 vitelline granulations encroach on this peripheral clear layer, and it 

 almost or entirely disappears. 



Returning now to a consideration of the smaller sphere, it is found 

 that the first polar globules differ considerably in size (25 /x to 40 fx), in 

 different eggs, even though the yolks be of uniform diameter. When 

 entirely detached, the polar globule is quite spherical, and remains for a 



* In another species of Limax I have seen, since the above was written, very prom- 

 inent pseudopodal elevations of the yolk at the animal pole toward the close of the 

 formation of the second polar globule (Fig. 95). Compare the explanation of the 

 figure. 



