120 MAMMALIAN FERTILIZATION 



spermy induced after hyperthermia in rats has also been reported 

 (Austin, 1955). Several pronuclei and sperm tails were shown 

 in rat eggs, but only male pronuclei were shown in rabbit eggs 

 ( Austin and Braden, 1953 ) . Whether this is due to a division of 

 the female pronucleus as the egg is fertilized at late stages in the 

 case of rabbit egg was not mentioned. Since the male pronuclei 

 in the rat egg "all take part in the formation of first cleavage 

 spindle," the second inhibition mechanism mentioned by Roth- 

 schild is not applicable in the rat egg. As for the tripolar spindle 

 observed in the rat egg (Austin and Braden, 1953, Fig. 9), it is 

 difficult to claim definitely it is a product of polyspenny because 

 a tripolar spindle among normal metaphase plates in a rabbit blas- 

 tocyst has been observed (Plate I, 4). 



In mammalian eggs, there is probably another mechanism to 

 prevent polyspermy, i.e., the reaction of the zona pellucida to 

 sperm penetration as reported by Braden et al. (1954). Accord- 

 ing to their observation, the penetrability of the zona pellucida 

 by sperm is not influenced by the entry of the first spermatozoon 

 into the rabbit egg, thus 62 sperms were found in the perivitelline 

 space of an egg. The number decreases after the entry of the 

 first sperm in the rat and mouse eggs, for the number of eggs 

 that contained more than one spermatozoon was of a lower fre- 

 quency than expected. The zona pellucida of sheep and dog eggs 

 precludes further penetration of sperm after the entry of the first 

 one because no extra sperms were detected in the perivitelline 

 space of fertilized eggs. Evidence of a change in the zona pellucida 

 has been reported by Smithberg ( 1953 ) , who observed that the 

 zona pellucida of unfertilized mouse eggs was invariably removed 

 by several proteolytic enzymes in less time than that of fertilized 

 eggs. Braden et al. ( 1954) also reviewed the published reports on 

 the number of spermatozoa within the eggs of various mammals. 

 It seems that the zona reaction to the sperm penetration in the 

 eggs of various species falls into the three categories previously 

 mentioned. The zona reaction to sperm penetration is one of the 

 important observations made in this field in recent years. Further 

 study to elucidate this mechanism would be of great interest. 



