116 EDWARD MCCRADY, JR. 



allantois, instead of extending out through the extra-embry- 

 onic coelom to fuse with the serosal chorion, remains in a 

 pocket of the yolk sac. 



Though this situation is remarkably different from that 

 found in most placental mammals, there is at least one of the 

 latter which is sufficiently like the opossum to be a useful 

 subject for comparison. The extra-embryonic membranes of 

 the 10-day rabbit (fig. 40, C) differ from those of the 10-day 

 opossum (A) in only two important points: 1) the allantois 

 is much further developed, and is fused with the serosal 

 chorion; and 2) the extra-embryonic coelom extends all the 

 way to the sinus terminalis, so that the yolk sac circulation 

 is completely separated from the surface of the vesicle. With 

 these exceptions the opossum and rabbit are strikingly similar 

 especially so in having the ventral two-thirds of the vesicle 

 composed only of yolk sac endoderm and ectoderm, and the 

 anterior third of the amnion composed only of proamnion. 



In later development the two species diverge considerably. 

 In the rabbit (D) the mesoderm invades the proamnion and 

 .splits, so that the yolk sac comes to be completely separated 

 from the amnion by extra-embryonic coelom. At the same 

 time the allantois extends its fusion with the chorion through- 

 out the serosal region, so that the allantoic vessels very liter- 

 ally replace the vitelline vessels. In direct contrast to this, 

 in the opossum it is the yolk sac which enlarges and displaces 

 the extra-embryonic coelom, so that almost the entire amnion 

 comes to be composed of proamnion, and the vitelline vessels 

 remain permanently in the chorion. In this situation the 

 allantois comes to be surrounded by the yolk sac, is prevented 

 from reaching the chorion, and serves only as an urinary 

 reservoir (see stage 33). 



Fig. 39 Reconstructions and sections of stage 29. The central reconstruction 

 taken from 17162 shows, in addition to the embryo proper, the constitution of 

 the amnion and area vasculosa. The sections are from 16154. The reconstruction 

 L shows, on a slightly smaller scale, the portion of the body cut off in the central 

 reconstruction. J shows the pharynx, lung buds, liver diverticulum, and dorsal 

 pancreatic diverticulum. M shows the actual size of the vesicle. 



