C. R. AUSTIN AND M. W. H. BISHOP 87 



sibly monotremes. In mammals it almost certainly gives rise to 

 polyploidy in the embryo (Austin and Braden, 1953b), and poly- 

 ploid embryos seldom go to term (Beatty, 1951). 



Mechanisms of Spermatozoon Penetration 



Passage of spermatozoa through the cumulus oophorus is pre- 

 sumably eflFected with the aid of the hyaluronidase that they are 

 known to carry (McClean and Rowlands, 1942; Fekete and 

 Duran-Reynals, 1943; Swyer, 1947a,b). Complete breakdown of 

 the cumulus mass is not a necessary prehminary to spermatozoon 

 penetration and normally does not occur until some time after 

 penetration. The function of hyaluronidase in fertilization, there- 

 fore, appears to be merely that of enabling the individual sperma- 

 tozoa to make paths for themselves through the matrix of the 

 cumulus (Lewis and Wright, 1935; Leonard, Perlman, and 

 Kurzrok, 1947; Austin, 1948a; Austin and Smiles, 1948; Blandau 

 and Odor, 1949; Bowman, 1951 ) . Subsequent denudation of the 

 egg by hyaluronidase, however, may be important in allowing 

 the eggs to start their journey through the tubes and in peniiit- 

 ting a ready gaseous exchange and elimination of metabolites by 

 removing the numerous actively metabolizing follicle cells. 



Fertilization in rabbits has been prevented by treating the se- 

 men used for insemination with hyaluronidase inhibitors or by 

 injecting the inhibitors into the vagina before insemination ( Pin- 

 cus, Pirie, and Chang, 1948; Parkes, 1953; Chang and Pincus, 

 1953; Parkes, Rogers, and Spensley, 1954). The results, however, 

 are inconclusive as all the hyaluronidase carried by the spermato- 

 zoa may not have been inhibited and to some extent the failure 

 of fertilization may have been due to the inhibition of other en- 

 zymes, including those involved in the metabolism of the sperma- 

 tozoa. 



The next step, the passage through the zona pellucida, is even 

 less well understood. In vitro, spermatozoa readily become at- 

 tached to the surface of the zona, particularly of rabbit eggs, and 

 this ability may be important in vivo in ensuring against escape 

 of the spermatozoon and in maintaining close application of the 

 spermatozoon head to pei*mit the action of a possible lytic agent 



