358 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1921. 



(figs. 10 and 12, pi. 4). As the vesicles grow they become more 

 evenly distributed in the uterus. Eventually they become greatly 

 crowded in the uteri and become jammed together under great 

 pressure. 



The vesicles are never attached to the uterus wall, there being no 

 placenta. Up to the time of birth it is easy to separate chorions 

 from the uterine mucosa with a brush, taking out the vesicles intact. 

 But the chorions are in very intimate contact with the mucosa and 

 follow it over every fold and down every furrow. Here the nutri- 

 tional exchange between mother and fetus takes place; and this is 

 quite sufficient for a few clays of fetal life. These relations are well 

 shown in figure 44, plate 9, which is taken from Selenka. 2 The 

 illustration shows two closely applied vesicles. In one the allantois 

 only is seen ; in the other the posterior end of an embryo also. The 

 chorion is represented as pulled away from the uterine wall at the 

 upper margin. 



NUMBER OF EGGS, EMBRYOS, AND TEATS. 



Hill has called attention to the overproduction of eggs in Dasyu- 

 rus ; and since the same phenomenon is seen in the opossum the fol- 

 lowing quotation from Hill's paper (1911) is of special interest: 



Dasyurus breeds but once a year, tbe breeding season extending over the 

 winter months — May to August. One remarkable feature in the reproduction 

 of Dasyurus is the fact that there is no correlation between the number of ova 

 shed during ovulation and the accommodation available in the pouch. The 

 normal number of teats present in the latter is six, though the presence of 

 one or two supernumerary teats is not uncommon ; the number of ova shed 

 at one period is, as a rule, far in excess of the teat number. I have, for 

 example, several records of the occurrence of from 20 to 25 esrgs, two of 28, 

 one of 30, and one of as many as 35 (23 normal blastocysts and 12 abnormal). 

 There can be little doubt that Dasyurus, like various other marsupials (e. g., 

 Perameles, Macropus, etc.), has suffered a progressive reduction in the number 

 of young reared, but even making due allowance for that, the excess in pro- 

 duction of ova over requirements would still be remarkable enough. Whether 

 this overproduction is to be correlated in any way with the occurrence of 

 abnormalities during early development or not, the fact remains that cleavage 

 abnormalities are quite frequently met with in Dasyurus. 



The average number of eggs shed by an opossum at ovulation is 

 22, or 11 from each ovary. Cases of 30 and 35 eggs are not un- 

 common ; and the records run up to 43 eggs. Indeed, if one ovary is 

 removed the other increases its activity to compensate for the loss. 

 Thus in four cases single surviving ovaries yielded 30, 40, 43, and 44 

 eggs. 



2 The opossum embryo at various stages has been beautifully and correctly figured by 

 Selenka in his Studien ueber die Entwickelungsgeschichte der Thiere, IV, Das Opossum, 

 Wiesbaden, 1S87. 



