FCETAL MEMBRANES OF OPOSSUM AND OTHER MARSUPIALS. 475 



outlined, the hind limb was still bud-like. The tail extended 

 somewhat beyond the hind limb; the cup of the eye was 

 backward in development, presenting a horseshoe appearance, 

 like that of a chick in the third day. Altogether by a com- 

 parison of the older embryos with some newly born opossums 

 found upon another female, I conjecture that the embryos were 

 about eight days old, and that the short period of intrauterine 

 development was about half over. A foetus of median size was 

 detached by a slight pressure of the needle, and the subzonal 

 membrane was found to be about 10 mm., or f of an inch in 

 diameter. Through this membrane the embryo could easily be 

 seen. There was an opaque disc-like area on the subzonal 

 membrane, and this v/as found to correspond to the partially 

 adherent yolk-sac, which was spread over about one third of 

 the inner surface of the membrane. When a portion of this 

 area was seen in profile a large number of minute villi 

 were at once noticed upon the surface of the subzonal 

 membrane, which was smooth elsewhere. The yolk-sac, as 

 in the kangaroos described by Professor Owen, had the figure 

 of a cone, the base attached to the subzonal membrane and the 

 apex at the umbilicus. At the edge of the area the yolk-sac 

 was folded back upon itself, as in fig. 1 (woodcut). The umbi- 

 lical stalk was wide. The attached area was covered with 

 capillary vessels, and circumscribed by the sinus terminalis ; 

 this united near one edge of the disc, to form a single vitelline 

 vein (PI. XXXIII, fig. 1), and the vitelline arteries were either 

 double or branched close to the embryo from a single trunk. 

 They were difficult to distinguish. 



The allantois was found in the various embryos in all stages 

 of development, two of which are represented in Plate XXXIII, 

 figs. 1 and 4). It arises, as in the Placentalia, just behind the 

 umbilical stalk, and the mesoblast and hypoblast could be readily 

 distinguished. In later stages it was a small sac with a wide 

 stalk. In the embryos which were examined no blood-vessels 

 could be detected, but they undoubtedly develop at a later 

 period. Compared with the yolk-sac the allantois was ex- 

 tremely small, nor was it in contact with the subzonal mem- 



