CESTROUS CYCLE IN THE GUINEA-PIG. 223 



ovulation unless the open vagina also contains what was described 

 as second or early third stage oestrous fluid. 



At that time the method of closure of the vagina following the 

 oestrus was not explained nor was its actual significance fully 

 appreciated. The vagina is now found to be closed by a remark- 

 able cellular membrane and in a very definite way. 



The external orifice of the vagina is crescentic in shape and 

 the urethral opening lies in front of it in the mid line. The 

 anterior and posterior lips of the crescent-shaped opening come 

 together, and a delicate epithelial membrane grows over the 

 opening and unites the lips. This occurs shortly after the heat 

 period in females that have not copulated and in those that have 

 copulated the closure follows the expulsion of the vaginal plug, 

 a process to be considered beyond. The closure begins at the 

 tips of the crescent-shaped opening and progresses toward the 

 midpoint. The lips do not approximate so intimately at the 

 midpoint and the membrane here seems to be under more 

 tension than at other parts, even after the entire orifice has closed. 

 The opening of the orifice by a tearing of the epithelial membrane 

 begins at the strained middle part and extends from there 

 laterally until finally the vaginal lips are freely separated. The 

 midpoint is, therefore, the last to close, and the first to open as 

 a general rule, although at times the opening may begin at either 

 side of the midline. 



The epithelium completely unites the lips of the vagina so 

 that nothing can escape from or enter into the vaginal lumen 

 without tearing this closure membrane. Such a membranous 

 closure of the vaginal orifice is unknown to us in any other 

 mammal. In many species the sides of the vaginal opening 

 may be approximated or cemented together by some hardened 

 fluid or secretion so that the lips are not readily pressed apart, 

 but a membranous growth closing the orifice after each heat 

 period is apparently unique. 



This membrane also completely closes the vaginal opening 

 throughout pregnancy and only becomes ruptured when the 

 vulva swells shortly before parturition. 



Such an obstruction or closure of the vaginal lumen at once 

 suggests the hymen of the human vagina. But this, of course, 



