THE EMBRYOLOGY OF THE OPOSSUM 



111 



body. Here on either side of the rhomboidal sinus they unite 

 in a very rich plexus through the region in which the pos- 

 terior limb ridge is forming (sections G and H). 



The allantois. Section H also shows a ventral diverticu- 

 lum from the hind-gut, which is the allantois. Its vasculariza- 

 tion is very rich, as the figure shows, and is derived from the 

 following sources. The arterial supply is from the dorsal 

 aortae. In the region of the mid-gut the aortae give off in- 

 numerable mesenteric arterioles (shown in the reconstruc- 

 tion). Just before reaching the hind-gut two exceptionally 





29 



Fig. 37 Sagittal sections of posterior end of embryos from stage 26 through 

 stage 29 to show cloacal membrane, caudal amniotic fold, extra-embryonic coelom, 

 hindgut, allantois, and the absence of any neurenteric canal. The section for 

 stage 26 is from 16146; for stage 27, 17141; for stage 28, 16150; and that for 

 stage 29 is a sagittal reconstruction from the cross sections of 16154. 



large branches, the vitelline arteries, descend into the walls 

 of the yolk sac, run caudad beneath the body (Gr and H), and 

 finally unite into one (I) which courses through the area vas- 

 culosa to the sinus terminalis (see also fig. 35, A and E). 

 Posterior to these, more arterioles are given off, and those be- 

 yond the posterior intestinal portal run into the walls of the 

 hindgut and thence into the allantois (H). The posterior wall 

 of the allantois (prospective ventral wall) is not surrounded 



