THE EMBRYOLOGY OF THE OPOSSUM 175 



It is probably correct to think of gestation in the opossum 

 as so consistent and precise in duration as to be defined in 

 terms of hours, not days; for when the whole period is so 

 short and development is so rapid, a difference of a few 

 hours makes a considerable difference in the stage of develop- 

 ment, and it is not likely that there can be any great irregu- 

 larity about the stage of development at birth. On the other 

 hand, all of the above estimates suffer from one defect. 

 Copulation in the opossum is not a momentary affair. It 

 sometimes lasts for 5 hours. When, then, is one to say copu- 

 lation occurred! My own notes always refer to the time when 

 intromission was first observed. If the time of fertilization 

 could be determined accurately, that would be the proper 

 starting point, but as long as our information about that is 

 only approximate, our knowledge of the length of gestation 

 must be only approximate. 



With the above reservation, then, all the evidence seems 

 to point to a rather sharply defined gestation period of about 

 12f days. 



Parturition. The mother sits on her haunches and licks the 

 vulva, the abdomen, and the inside of the pouch. Contrac- 

 tions of the uterus probably force the young into the vaginal 

 cul-de-sac. The chorions, which are all fused into a single 

 mass and intimately associated with the mucosa of the uterus 

 by complex folds, are left in the uterus. The amnion and 

 fragments of the allantois go with the young into the vaginal 

 cul-de-sac. Contractions of the maternal abdominal wall then 

 cause the lateral vaginal canals, which are bent upon them- 

 selves, to become completely occluded, and the increased intra- 

 abdominal pressure can be relieved at only one point the 

 pelvic canal, which leads to the outside. The posterior wall 

 of the vaginal cul-de-sac is thin and lies in contact with the 

 anterior wall of the urogenital sinus, the two together forming 

 a delicate membrane across the pelvic canal (fig. 5). This 

 membrane ruptures under the increased pressure from the 

 inside, and an artificial, but direct, passage the pseudo- 

 vaginal canal is formed. The young pass through this into 

 the urinogenital sinus, and thence to the outside. 



