BREEDING HABITS OF THE OPOSSUM — HARTMAN. 351 



and in a few instances uterine eggs were recovered as early as five 

 days after removal of the young. Such findings might give color 

 to the statement of some authors that a single female may carry two 

 litters of different ages at one and the same time. But for reasons 

 which will be stated below such " superf etation " in the opossum is 

 unlikely if not impossible. 



THE CESTROUS CYCLE. 



The writer has also followed some scores of individuals through 

 the oestrus cycle and has determined that there is no fundamental 

 difference between the opossum and the higher mammals in the 

 various phenomena accompanying the cycle. For several seasons 

 the changes occurring in the mammary gland as diagnosed by pal- 

 pation were followed. As heat approaches the glands swell and 

 become firm ; and after ovulation, whether pregnancy ensues or not, 

 the mammary gland continues its rapid swelling and turgescence 

 for five or six days. 



This statement has two important implications. First, ovulation 

 in the opossum is spontaneous, that is, copulation is not necessary 

 for the dehiscence of the follicles as is the case in the cat and 

 probably some other mammals. Second, whether the eggs are fer- 

 tilized or not, the reproductive organs develop up to a certain point, 

 probably under the stimulus of the corpora lutea; and this state- 

 ment holds for the uteri, the lateral vaginal canals and the vagina 

 as well as for the mammary glands (cf. figs. 8 and 9, pi. 3). This 

 striking development of the organs outside. of pregnancy has been 

 termed by Professor Hill the stage of " pseudopregnancy." This 

 stage is pronounced in the marsupials ; but as Ancel and Bouin and 

 Long and Evans have shown the stage is also manifested to some 

 degree in the rabbit and the rat. 



If pregnancy ensues, the uteri and the mammary glands continue 

 their growth and development, the uteri till birth, the mammary 

 glands for about two months (fig. 45, pi. 10). But pseudopreg- 

 nancy may be distinguished from pregnancy after the sixth day by 

 regressive changes in the mammary glands, which become flaccid 

 and gradually thinner so that several days before the next heat they 

 are almost as thin as in the resting stage. 



Thus the cycle was established and found to be about a month 

 in duration. 



For the last two seasons the cycle was also followed by the aid of 

 Stockaid and Papanicolaou's method of studying smears of the cell 

 content of the vagina. By this method, also, a distinct rhythm was 

 observed quite comparable with that shown by Stockard and 

 Papanicolaou for the guinea pig and by Long and Evans for the rat. 



