100 THE MAMMALIAN EGG 



it is suggested that when the spermatozoon reaches the egg the 

 acrosome is removed, laying bare the perforatorium (Austin and 

 Bishop, 1958b, c), the probable function of which has already been 

 discussed (p. 92). 



It is tempting to argue that the capacitation change of the mam- 

 malian acrosome is analogous to the 'acrosome reaction' exhibited 

 by spermatozoa of several invertebrate species (see Dan, 1956; 

 Col win and Col win, 1957; Franzen, 1958). The invertebrate 

 acrosome reaction is provoked by substances in the jelly coats 

 covering eggs or diffusing from the eggs into the medium; it finds 

 expression in the protrusion of an acrosome filament and the release 

 of lytic agents, both processes evidently making possible the entry 

 of the spermatozoon into the egg. The reason for drawing this 

 parallel is to support the suggestion that the normal capacitation 

 process may turn out to be a reaction of the spermatozoon, not to 

 tubal or uterine secretions, but to substances in or emanating from 

 the cumulus masses as they lie in the Fallopian tube. As yet, how- 

 ever, it has not been found possible to obtain the mammalian 

 acrosome reaction by merely placing spermatozoa and cumulus 

 masses together in vitro, and so it is necessary to suppose that capacita- 

 tion within the female tract involves also a preliminary phase in 

 which substances present in the ejaculate and exerting an inhibitory 

 effect are removed from the spermatozoa. 



If the ideas just set out on the passage of spermatozoa through 

 the cumulus are substantially true, the cumulus, in those animals in 

 which it persists, could be regarded as constituting another line of 

 defence against the danger of polyspermic fertilization, by providing 

 a hindrance to sperm passage which individual spermatozoa may 

 well vary in their ability to overcome. On the other hand, it can 

 also be argued that the cumulus improves the chances of fertilization 

 by providing a larger target for spermatozoa to encounter and by 

 orientating the spermatozoa towards the egg, through the radial 

 arrangement of the follicle cells. Perhaps, these two functions 

 would not necessarily be conflicting. 



Mucin Coat of the Rabbit Egg 



The mucin coat, originally designated the 'albumen' coat and 

 formed of material secreted by the epithelium of the rabbit Fallopian 

 tube, becomes deposited in the final stages of disintegration of the 

 cumulus, and often imprisons a few coronal cells. It shows distinct 



