20 EDWARD MCCRADY, JR. 



Unmated females show a regular dioestrous cycle of about 

 28 days, as indicated by urinogenital sinus smears, beginning 

 in January and ending in September. The last 3 months of 

 the year constitute an anoestrum. Ovulation does not occur 

 during lactation. Pregnancy lasts nearly 13 days, and lacta- 

 tion from 70 to 80 days, so that certainly two litters, and 

 possibly three, may be borne by one female in a year. My 

 own experience seems to indicate that the anoestrum may be 

 somewhat longer in Pennsylvania during cold winters, and 

 probably two litters is a maximum for this part of the country. 

 The graafian follicles usually burst during or very near the 

 time of oestrus. Hartman ('28, p. 162) records one observa- 

 tion which he interprets as evidence that copulation sometimes 

 takes place too late for fertilization (i.e., after the ova have 

 become enclosed in too much albumen, or have begun to de- 

 generate). If so, the implication is that in the opossum 

 ovulation is not dependent upon any stimulus from coitus or 

 related to oestrus in any invariable way. The cleavage stages 

 or early vesicles occasionally found at abnormally long inter- 

 vals after coitus may be interpreted as confirmatory evidence. 

 Hartman 's no. 298 is a case in point. The vesicles he found 

 in the first operation 6 days after coitus belong to my stage 10 

 and are approximately 2 days late. In the second operation, 

 9 days after coitus, he found vesicles of my stage 21. This 

 represents a perfectly normal rate of development between 

 the two operations, so the whole batch of eggs must have 

 started development 2 days late. This means they would have 

 entered the uterus uncleaved about 3 days postcoitum. On 

 March 1, 1937, I obtained eleven uncleaved uterine ova 

 (no. 137) in perfect condition from a uterus opened exactly 

 73 hours postcoitum. In this case the female had been isolated 

 immediately after the observed copulation, so the possibility 

 of a second copulation 2 days later is ruled out. 



