92 PRELIMINARIES TO FERTILIZATION 



1956). The rate of its development appears to decrease as the 

 egg ages after ovulation (Austin and Braden, 1953a,b; Braden 

 and Austin, 1954c), and this effect is promoted by local heat 

 (Austin and Braden, 1954b) and by hyperthermia (Austin, 

 1955a, 1956a; Austin and Braden, 1956). Both aging and heat 

 have long been know^n to increase the incidence of polyspermy in 

 invertebrate eggs (Wilson, 1928). It has not yet been found pos- 

 sible to express quantitatively the normal rate of development of 

 the block in mammals, but estimates have been derived for sea 

 urchin eggs. There is, it seems, a fast partial block covering the 

 egg in less than two seconds and a slower complete component 

 taking about one minute (Rothschild and Swann, 1952; Roths- 

 child, 1954). 



An early reaction to the penetrating spermatozoon shown by 

 certain invertebrate eggs consists in the formation of a fertiliza- 

 tion cone. Many years ago Asterias (starfish) eggs were observed 

 to develop a filament that moved out to make contact with the 

 nearest spennatozoon and appeared to assist its entry into the 

 egg (Fol, 1877, 1879; see also Colwin and Colwin, this volume). 

 Since then other forms of fertilization cones have been described 

 in different species; in some animals the projection persists after 

 spermatozoon entry, and in others it is withdrawn before this 

 event ( Chambers, 1933 ) . Recent observations on rat, mouse, and 

 hamster eggs show that an analogous elevation develops in the 

 mammalian egg as the spermatozoon head is absorbed through 

 the vitelline surface; it persists for a short time thereafter (Aus- 

 tin and Braden, 1956). 



It may be seen, therefore, that two of the earliest reactions 

 shown by the mammalian egg to spermatozoon penetration tend 

 to prevent the entry of other spermatozoa. Both reactions are ap- 

 parently evoked by contact of the spermatozoon head with the 

 vitelline surface, but their relative importance varies between 

 species in a complementary manner. 



Conclusions 



The evolution of internal fertilization brought with it numerous 

 advantages arising from the complete avoidance of the external 



