THE EMBRYOLOGY OF THE OPOSSUM 119 



Before the four folds meet, mesodenn invades the pro- 

 aranion for a short distance ahead of the amniopore, so that 

 the latter comes to be surrounded by extra-embryonic coelom. 

 When the amniopore then closes (at the site indicated in the 

 reconstruction, fig. 39) the partition or stalk which tends to 

 form is quickly resorbed and amnion and chorion become 

 separated from each other by extra-embryonic coelom 

 throughout the region beneath the serosal chorion. However, 

 as the mesoderm has invaded the proamnion for only a short 

 distance anterior to the amniopore, its anterior extremity 

 is marked by a two-layered membrane still connecting the 

 amnion with the chorion. This membrane is composed of 

 somatic mesoderm and yolk sac endoderm. If it were to break 

 down, coelom and yolk sac would open directly into each 

 other; but this never occurs. If traced laterally along this 

 membrane, the somatic mesoderm will be found to meet the 

 splanchnic mesoderm along a line marked conspicuously by 

 the large omphalomesenteric vein coming in from the area 

 vasculosa. As this vein always marks their line of junction 

 it will be an interesting landmark by which to follow the 

 later changes in proportion between the yolk sac and the extra- 

 embryonic coelom and the corresponding parts of the amnion. 

 Tracing the splanchnopleure, which forms the lateral wall of 

 the extra-embryonic coelom, caudally from the omphalo- 

 mesenteric vein, we find that it meets its fellow of the oppo- 

 site side in the midline behind the tail bud; and that their 

 line of junction is marked by the large vitelline artery which 

 follows it out to the area vasculosa (fig. 39, H, I, K). There 

 are thus three large vessels running between the embryonic 

 body and the chorion and marking the corners of a roughly 

 prismatic extra-embryonic coelom. The membranes which 

 connect these three vessels to each other also connect the 

 embryonic body to the chorion, and form a sort of prismatic, 

 hollow, body stalk. These membranes are two-layered. The 

 outside layer on all sides is yolk sac endoderm. The inside 

 of the anterior face (i.e., the one between the two veins) is 

 somatic mesoderm. The inside of the other two faces is 



