THE MORPHOLOGY OF FERTILIZATION 53 



vitelline membrane beneath which is an alveolar cortical 

 layer (Fig. 2a); the large germinal vesicle is central in 

 position; the protoplasm contains numerous yolk spheres 

 and a broad equatorial band of refringent oil drops. 

 When insemination takes place, a large number of sper- 

 matozoa become attached to the ovum, if the sperm is 

 present in excess. In about two or three minutes all 

 spermatozoa, with the exception of one, which is alone 

 concerned in the subsequent fertilization, begin to be 

 carried away from the surface of the egg by an outflow 

 of jelly, derived from the alveoli of the cortical layer 

 which are gradually emptied, thus establishing a perivi- 

 telline space crossed by the delicate protoplasmic walls 

 of the original alveoli. A transparent fertilization cone 

 then arises from the inner wall of the perivitelline space 

 opposite the attached spermatozoon and extends across 

 the space until it touches the membrane at the point 

 of attachment of the spermatozoon (Fig. 2c). The per- 

 foratorium of the spermatozoon pierces the vitelline 

 membrane and becomes imbedded in the cone. These 

 phenomena occupy about fifteen minutes. The cone 

 then gradually flattens out, but stained sections show 

 that it persists as a modified area of the protoplasm. 

 For about thirty minutes more no obvious changes 

 occur. The head of the spermatozoon then disappears 

 rather abruptly into the ovum, and the tail and middle 

 piece are left behind on the surface of the vitelline 

 membrane. 



Stained sections show the details of the final penetra- 

 tion of the sperm head very beautifully (Fig. 3). The 

 complex made up of the head of the spermatozoon 

 and the fertilization cone act as a unit. The cone 



