STRUCTURE AND FUNCTION IN MAMMALIAN EGGS 93 



vitelline surface causes the release of a substance which diffuses 

 through the perivitelline fluid and renders the zona pellucida im- 

 permeable to spermatozoa (Fig. 71) (Austin and Braden, 1956). 

 This theory invokes a system that, as Rothschild (1956) points out, 



Fig. 71 

 Diagrams of rat eggs to show how the zona reaction is believed to spread out in relation 

 to the point of sperm attachment on the surface of the vitellus. (From Austin and Bishop, 

 1957b.) 



is widespread in the animal kingdom : the arousal by sperm penetra- 

 tion of a reaction that is propagated over the egg surface and is 

 associated with the release of an agent that has the function of 

 rendering a membrane impermeable to spermatozoa. In sea-urchins, 

 the response to contact by the fertilizing spermatozoon involves the 

 sudden expansion ('explosion') of cortical granules, the contents of 

 which apparently unite with the vitelline membrane converting it 

 into the sperm-impermeable fertilization membrane (Fig. 72). 

 Elevation of the fertilization membrane is thought to be due to the 

 osmotic effect of colloids released in the reaction. Cortical granules 

 of a different kind have been described in Nereis and these are packed 

 in regularly arranged alveoli; the reaction to sperm penetration is 

 also different in detail but presents the common features of cortical 

 propagation, release of specific substances (which produce a volum- 

 inous jelly coat in this instance), and alteration of the vitelline 

 membrane (Costello, 1949). Fish-egg alveoli do not resemble those 

 of Nereis in appearance, nor the cortical granules of sea-urchin eggs, 

 but here again there is a propagated change and the alteration of a 

 membrane ('hardening' of the chorion) evidently under the action 

 of substances released from the alveoli (see Rothschild, 1958; Zotin, 

 1958). 



