C. R. AUSTIN AND M. W. H. BISHOP 89 



on or near the leading surface of the head with which the sup- 

 posed zona lysin could be associated (Bowen, 1924; Green, 1940), 

 for example, the "vesicle" on the head of the rat spermatozoon 

 (Austin and Sapsford, 1952). It is of interest, too, that a hereditary 

 deformity of the "acrosome" in bull spermatozoa has been found 

 to be associated with sterility ( Hancock, 1949, 1953 ) . The extrac- 

 tion of a lytic agent from the anterior end of the head of Disco- 

 glossus ( toad ) spermatozoa has been reported ( Parat, 1933 ) and 

 similar observations have been made with Megathura (keyhole 

 limpet) spermatozoa (Tyler, 1949) and Mytilus (mussel) sper- 

 matozoa (Wada, Collier, and Dan, 1956). 



After passing through the zona pellucida the fertilizing sper- 

 matozoon projects into the perivitelline space and because this is 

 small the spermatozoon head soon makes contact with the vitellus 

 or cytoplasmic part of the egg. Contact appears to evoke a reac- 

 tion in the vitellus so that attachment is formed between the 

 spermatozoon head and the surface of the vitellus (Austin and 

 Braden, 1956). The final phase of spermatozoon penetration, the 

 entry into the egg cytoplasm, takes place in a manner suggesting 

 that it is chieHy a function of the vitellus (Austin, 1951). The 

 spermatozoon is absorbed much as a food particle is absorbed 

 by an amoeba, or, as Loeb (1917) pointed out, in a manner re- 

 sembling phagocytosis. Compared with penetration through the 

 zona, entry into the vitellus is comparatively slow, the sperma- 

 tozoon head being attached to the vitelline surface but essentially 

 within the perivitelline space for about half an hour in the rat, 

 mouse, and hamster (Austin and Braden, 1954a; Austin, 1956c). 

 Thereafter, part of the spermatozoon midpiece or tail may still be 

 seen in the perivitelline space or protruding through the hole in 

 the zona at times up to several hours after the entry of the head. 



Attachment of the spermatozoon head to the vitelline surface 

 appears to be an essential preliminary to entry into the vitellus. 

 Supplementary spermatozoa remain free within the perivitelline 

 space. Probably anything altering the normal state of either sper- 

 matozoon or vitellus may prevent attachment from taking place. 

 It has been reported that unfertilized rabbit eggs with a sperma- 

 tozoon still in the perivitelline space, well after the normal time 



