350 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



referring to the aplacental marsupials. Again, as is well known, 

 when once attached to a teat, the pouch young must remain attached 

 until ready to leave (about two months) ; hence only the occupied 

 teats are suckled by a given litter of young, whereas in other mam- 

 mals first one, then another, hence all of the teats are used and all 

 develop. Here, then, a simple observation is all that is necessary to 

 show that suckling is the stimulus which causes the glands to continue 

 their growth during lactation. This purely local effect is seen in 

 figure 45, plate 10, which represents a section through four occupied 

 and two unoccupied (a) mammary glands. 



These examples must suffice to show that a study of a " different " 

 animal is not only interesting to the lay public because it is rela- 

 tively rare, but also proves to be of value to the physiologist because 

 of the sidelights which its study may throw upon the theoretical 

 phases of his subject. 



THE DIAGNOSIS OF PREGNANCY IN THE OPOSSUM. 



There are certain other observations which have facilitated the 

 collection of embryos. It has been found possible to diagnose preg- 

 nancy with practical certainty and even the stage of pregnancy with 

 a fair degree of accuracy. This method is by palpation of the mam- 

 mary glands. Towards the first of the breeding season the skin of 

 the pouch containing the resting glands begins to swell and to take 

 on a certain firmness and turgor. By feeling of the animals on 

 successive days one may select from among hundreds only the preg- 

 nant ones, leaving the others. QEstrus may, however, also be diag- 

 nosed by Stockard and Papanicolaou's vaginal smear method, as 

 will be seen below. 



THE OPOSSUM IS POLYCESTROUS. 



The opossum is polycestrous ; that is, it has more than one oestrus 

 or heat period during a given breeding season. A female may have 

 two (or perhaps, in rare instances, three) litters a year, and this 

 plurality of litters alone classifies it as a polycestrous animal. If, 

 however, at the first oestrus impregnation does not take place or 

 if the embryos die in utero, which seldom happens, or if the pouch 

 young are removed or lost, the female will sooner or later come into 

 heat again. The writer has removed pouch young many times. The 

 interval between such removal and the onset of a new oestrus is 

 found to be extremely variable, depending as much on the condition 

 of the female as on any other factor. In general, the older the 

 pouch young, the shorter the interval; for with the growth of the 

 suckling young the ovarian follicles also develop. Indeed, there are 

 several records of lactating females almost at the stage of oestrus 



