THE EMBRYOLOGY OF THE OPOSSUM 23 



of the mixture of lateral vaginal canal fluid and semen begins 

 within a very short time. The spermatozoa which do not 

 escape quickly into the uterus are then imprisoned in a coagu- 

 lated mass the vaginal plug. Twenty-four hours later the 

 vaginal plug has disappeared. 



Fig. 4 Ventral view of male external genitalia. The penis is caudal to the 

 scrotum, and its glans is bifurcate. During detumescence it retreats out of 

 sight to a position similar to that of the clitoris. 



Lewis ('25) studied the bacteria present in these canals at 

 different stages of the oestrous cycle. During the anoestrum 

 he found their number to be reduced to a minimum. During 

 the prooestrum the numbers, both of individuals and of 

 species, increase rapidly. The maximum is attained at 

 oestrus. As all of the species identified, of which there are 

 about a dozen, are well-known saprophytes, he concluded that 

 they are responsible for the removal of the vaginal plug and 

 for the general cleaning of the canals during the dioestrum 

 and anoestrum. 



