174 EDWAED MdCRADY, JR. 



Selenka did not have the good fortune to observe parturi- 

 tion, but by removing the two uteri at different times and at 

 known intervals after copulation he obtained accurate in- 

 formation about the timing of twelve of the thirty-five stages 

 I have defined. According to my stage numbers the stages 

 he thus located are 13, 14, 18, 22, 24, 27 (slightly abnormal 

 specimen, but accurately timed), 28, 29, 32, 33, 34 and 35. His 

 timing is accurate only when computed from copulation, which 

 can be done from the data he gives; but the ages as he re- 

 corded them (in terms of hours after the beginning of 

 cleavag'e) are inaccurate on account of the mistake he made 

 about the time of cleavage. His cleavage stages are all de- 

 generating unfertilized ova, as already pointed out by Hart- 

 man. The cases in which he described internal structures ac- 

 curately are noted in the appropriate parts of this text, but 

 in many stages he omitted the internal anatomy. 



Though he did not observe parturition, on one occasion he 

 found a litter of young in the pouch 13 days after an observed 

 copulation. And on another occasion he found embryos still 

 in the uterus 12 days and 18 hours after an observed copula- 

 tion. He concluded that birth must take place about 12 days 

 and 21 or 22 hours after copulation. 



Hartman ( '28) using the same method was able to assemble 

 a large number of stages, the intervals between which were 

 accurately known. But in only one case did he observe par- 

 turition, and in this case, unfortunately, he had not observed 

 copulation. So his estimate of the length of gestation was 

 based upon a known maximum the age of the youngest litter 

 he found in the pouch and corroborative evidence derived 

 by totalling the known intervals between a large number of 

 separate stages. In this way he arrived at a figure similar 

 to that suggested by Selenka i.e., in the neighborhood of 

 13 days. 



I have found a litter in the pouch 12 days and 19 hours after 

 an observed copulation. In conjunction with Selenka 's case 

 of unborn embryos at 12 days and 18 hours, this would seem 

 to locate the time of parturition pretty accurately at about 

 12 davs and 18^ hours. 



