352 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



At the onset of oestrus, when the ovary contains ripe follicles, the. 

 vagina is greatly swollen and the lateral vaginal canals are distended 

 to enormous proportions by a clear, thin, stringy mucus. Indeed 

 this swelling" of the lateral vaginal canals as the ovarian follicles 

 reach medium to full size is a most striking phenomenon — the canals 

 may even greatly exceed the uteri in size (fig. 7, pi. 2). After 

 ovulation they rapidly retrogress and become filled with a dry 

 cheesy mass of epithelial debris. Since there is probably a con- 

 siderable delay between cestrus (copulation) and ovulation, it is 

 likely, as Hill has suggested, that these canals serve as receptacula 

 seminis. 



In studying the vaginal smear of the opossum there is one com- 

 plication which somewhat masks the picture shown. In the higher 

 mammals the presence or absence of white blood cells is diagnostic 

 for certain stages in the cycle; but in the opossum the cloaca and 

 the vagina are often filled with corpuscles from the anal glands (scent 

 glands?) ; and since these corpuscles simulate white blood cells it 

 has thus far been impossible for us to use the presence or absence 

 of these cells from the smears to diagnose any particular stage of the 

 dicestrous cycle. 



To recapitulate, then, if a female opossum is kept isolated from 

 males there recurs throughout the breeding season a succession of 

 cestrous cycles about a month in length. These cycles may be fol- 

 lowed without injury to the animal by simple palpation of the mam- 

 mary glands or by the study of vaginal smears. 



THE BREEDING SEASON. 



The breeding season of the Virginia opossum begins in January at 

 Austin, Tex., and probably several weeks later in the North. A few 

 individuals may come into heat in the first week of the year, but 

 more enter this condition the second week. In the third week the 

 season is at its height; hence the embryologist desirous of securing 

 eggs and embryos would best time his collection during the last week 

 in January and the first week in February. The prevailing weather 

 seems to have no effect on the onset of the breeding season. 



By the middle of February most females captured have young in 

 the pouch. But late in the spring and in the summer there is great 

 irregularity in the condition of the females, so that one may capture 

 females with small young in the pouch any time between May and 

 September. Many reasons may be advanced for this variability: 

 accidents to the mother resulting in early loss of the young; vari- 

 ability in weaning age ; condition of the female, the robust reproduc- 

 ing faster than the weak; age of the animals, the very youngest 

 "yearlings" and the oldest multiparas being the latest to come into 

 heat. 



