118 MAMMALIAN FERTILIZATION 



divide, develop, and differentiate to an early stage and fail later. 

 Probably the contribution of the male element is more important 

 for more advanced development, differentiation, and adaptation. 



Sperm Penetration 



The sperm penetration of the rabbit egg has been observed 

 by Pincus ( 1930 ) , who was under the impression that there was 

 a slight bulging of the ooplasm at the point of sperm entry. The 

 penetration of the rat egg was studied recently by means of phase- 

 contrast microscopy. According to Austin (1951a), the penetra- 

 tion of the rat egg is a very rapid process, taking no more than a 

 few minutes at most for the head to pass through the zona pel- 

 lucida. When the spermatozoon passes through the zona of the 

 rat egg it leaves a slit or potential hole. The entry of the sperma- 

 tozoon into the ooplasm has been observed on several occasions. 

 The spermatozoon may pass straight through the zona into the 

 ooplasm or it may remain for a variable period in the perivitelline 

 space. The head sinks into the vitellus and, the rest of the sper- 

 matozoon, being motionless, is gradually taken in. According to 

 Austin (1951a) the penetration into the vitellus appears to be 

 a function of the vitellus itself, and there must be some property 

 of the head that results in its being absorbed into the vitellus. In 

 a study of the spemi penetration of the rat egg, Blandau and 

 Odor ( 1952 ) did not mention the slit in the zona pellucida, but 

 they stated that the forward progression of the fertilizing s )er- 

 matozoon into the ooplasm is discontinuous and is dependent 

 upon a particular type of undulating movement of the tail which 

 forces the head forward a distance of 10 to 20 microns at a time. 

 They have also observed accessory spermatozoa penetrating into 

 the ooplasm in which the fertilizing spermatozoon has already 

 assumed its final position. The forward movement of accessory 

 sperm, however, is much more continuous and uninterrupted and 

 on one occasion, an accessory spermatozoon was seen to enter 

 and leave the ooplasm a number of times without visible sisjns 

 of decreased activity. But in no instance was the retention of the 



