96 THE MAMMALIAN EGG 



found to contain two rat spermatozoa in the peri vitelline space 

 (Leonard and Perlman, 1949; A. K. Tarkowski, A. W. H. Braden, 

 R. G. Edwards and C. R. Austin, unpublished data). In the great 

 majority of these experiments, the foreign spermatozoa achieved the 

 site of fertilization, often in numbers that were well within the 

 normal range. Provisionally, it is suggested that the zona pellucida 

 is resistant to penetration by spermatozoa of other than closely 

 related species, though the possibility cannot yet be excluded that 

 it is primarily the process of capacitation that is involved in this 

 distinction. 



Another phenomenon in which the zona pellucida possibly plays 

 a role is that of selective fertilization. Braden (1958b) showed that 

 the fertilization efficiency of spermatozoa is influenced by the genetic 

 constitution of the male, and later (Braden, 1958c) concluded that 

 the chances of egg penetration by spermatozoa could be influenced 

 by a single genetic locus (the T locus). Evidence showed that 

 spermatozoa carrying a t allele were in some way handicapped for 

 the task of traversing the utero-tubal junction (Braden and Glueck- 

 sohn-Waelsch, 1958), but more recent information indicates that the 

 transmission ratio of t and T is also influenced by the genotype of 

 the egg, and tins appears to mean that the ease of penetration of 

 eggs differs under genetic control (Braden, i960; Bateman, i960). 

 The mechanism is as yet unknown but may well involve properties 

 of the zona pellucida. 



Cumulus Oophorus 



The cumulus oophorus or membrana granulosa is the mass of 

 cells that comes to surround the oocyte as the follicle grows. At 

 ovulation, the egg passes to the Fallopian tube still surrounded, in 

 most animals, by the cumulus; in the opossum, the egg is said to 

 reach the Fallopian tube already freed of the follicle cells. In other 

 animals, the investment persists for very variable periods of time. 

 The cumulus in the sheep, cow, horse and man breaks up readily 

 and sperm penetration is considered normally to be into eggs free 

 of cells (denuded eggs). In the rodents, the rabbit and the pig, 

 denudation occurs during the period of sperm penetration or shortly 

 thereafter. Cat and dog eggs retain a coating of follicle cells even 

 after the first cleavage division. 



The cumulus oophorus is made up of large numbers of follicle 

 cells embedded in a transparent jelly-like matrix (Fig. 74). The 



