224 CHARLES R. STOCKARD AND GEORGE N. PAPANICOLAOU. 



is quite a different structure in its origin as well as in its later 

 history. The closure membrane in the guinea-pig not only 

 exists in the young immature animal but is regularly destroyed 

 before and reformed after every heat period that takes place 

 during the life of the female. The formation or growth of this 

 membrane might also be compared in some respects to the 

 membranous growths tending to extend across and close the 

 pharynx and other canals under pathological conditions. 



The membrane is thin and delivate in structure and when 

 stretched by slightly pressing apart the lips of the vagina with the 

 ringers it is seen to be almost transparent, the outline of the 

 vaginal lumen showing through. The closure membrane is of 

 the same glossy appearance as is the surface epithelium covering 



o 



the region of the vaginal lips with which it is continuous. It is 

 composed simply of stratified squamous epithelium which has 

 grown from the borders of the lips over the orifice and contains 

 no vessels or blood. 



When the membrane is torn or broken by accident during the 

 dioestrum, or period of sexual rest, it reforms sometimes within 

 a day, or within a few days, and remains until the beginning of 

 the new period of heat or oestrus. A recognition of this mem- 

 brane is then a great convenience in determining the onset of the 

 oestrus in a group of female guinea-pigs. Daily smears of the 

 vaginal fluid are not now necessary to find when the oestrus is 

 about to begin in animals examined for the first time and whose 

 rhythm is therefore unknown. 



Although the presence of the closure membrane is a definite 

 aid in recognizing the condition of the cestrous cycle, it must be 

 remembered that this membrane often persists up to the first 

 stage of cestrus, at which time the lumen of the vagina is filled 

 with a mucous fluid and first stage cells. This is actually the 

 "heat" time and the normal moment for copulation as we shall 

 explain below. When the closure membrane still persists until 

 the vaginal lumen is so filled it may be distended and rounded 

 out resembling a blister membrane on the point of bursting. 

 Puncturing this the vaginal fluid oozes out through the break. 

 As a general rule, however, the vulva becomes inflamed and very 

 slightly swollen immediately before cestrus and the stretched 



