8 



E. E. JUST. 



thus diminishing the positive reaction of the cortex between two 

 sperm points. Often, of course, even in heavy insemination a 

 single spermatozoon reaches the egg at some one point ahead of 

 others; in this case the egg is monospermic and so behaves. 

 The observer is indeed struck with the rare occurrence of poly- 

 spermy even with heavy insemination. 



When a spermatozoon becomes attached not only the pene- 

 tration of other sperm but also their fixation to the vitellus 

 depends upon the degree of penetration of the .attached sperm or 

 perhaps rather on the rate at which the "wave of negativity" is 

 propagated around the egg. Thus, at the beginning of penetra- 

 tion, sperm nearby cannot become attached to the vitellus; 



ii 



9 10 



FIG. 9. Two minutes after insemination. Membrane is fully rounded, equidis- 

 tant at all points from the now spherical egg. 



FIG. 10. About same time as in Fig. 9. Cone is clearly visible. 



FIG. ii. Ten minutes after insemination. Egg with hyaline plasma layer 

 formed 



those farther removed may become attached, lashing back and 

 forth very vigorously. The heads of those still farther away but 

 not far enough beyond the area of decrement actually to be 

 engulfed are likewise affixed and present a swollen appearance. 

 When the membrane lifts off these affixed sperm are carried with 

 it. 



The Echinarachnius egg, then, would seem to be unusual, since 

 a gradient of susceptibility to the sperm may actually be demon- 

 strated in the cortical changes following sperm entry. 



After elevation of the membrane in Echinarachnius egg a cone 

 (Fig. 10) of clear cytoplasm forms at the point of sperm entry; 

 sectioned eggs show the sperm within where rotation of the head 

 may begin. This cone 1 disappears; within some ten to fifteen 



1 The cone and the behavior of sperm that are attached to the cortex but do not 

 enter constitute important checks on the observation of the point of sperm entry. 



