M. C. CHANG 119 



accessory spermatozoon within the ooplasm observed. This is 

 an extraordinary phenomenon, if these ova are not degenerating, 

 and it needs to be confirmed. 



The modified fertihzation cone described by Pincus in the 

 rabbit egg has not been observed in the rat egg, and the small 

 bit of clear cytoplasm enclosing the fertilizing sperm head in cer- 

 tain rat eggs is considered not a fertilization cone but a result of 

 shrinkage of the vitellus (Blandau and Odor, 1952). 



Austin and Blandau and Odor agree that several spermatozoa 

 penetrate into the zona pellucida of the rat eggs, 23 in one of 

 Austin's eggs, but they are in disagreement on the manner of 

 sperm entry into the ooplasm. According to the writer's experi- 

 ence with fixed rabbit eggs, the fertilizing spermatozoon is on the 

 very edge of the ooplasm before it changes its shape. If this is 

 true in the rat egg, it is hard to distinguish whether the sperm 

 is absorbed by the vitellus or pushed into the vitellus; maybe 

 both play a part especially considering the viscous nature of 

 the ooplasm. 



One point of interest is that Austin and Blandau and Odor 

 have observed the penetration of spermatozoa into the vitellus of 

 rat eggs in vitro, but as yet no one has observed penetration 

 through the zona pellucida. Since spermatozoa were never seen 

 entering more than halfway through the zona pellucida during 

 a study of fertilization of rabbit eggs in vitro (Chang, unpub- 

 lished ) , it seems that the penetration of spermatozoa through the 

 zona pellucida is more dependent upon the participation of the 

 Fallopian tube than is the penetration of ooplasm. 



The problem of polyspermy has been reviewed by Rothschild 

 ( 1954 ) . In many species of animals there is a rapid block at the 

 egg surface after the entry of the fertilizing spermatozoon. In 

 others (birds, salamanders, etc.) many sperm may enter but there 

 is an inhibition of supernumeraiy sperm nuclei to prevent 

 their uniting with the female pronucleus. After a review of 

 the literature on polyspermy in mammalian eggs, Austin and 

 Braden (1953) report that when fertilization is delayed after 

 ovulation, polyspermy occurs in the rabbit and rat eggs. Poly- 



