THE EMBRYOLOGY OF THE OPOSSUM 153 



has shortened the distance between the cephalic and cervical 

 flexures and lifted the head somewhat away from the chest; 

 the mouth is sometimes open and the tongue protruding; the 

 allantois has attained its maximal size with a diameter equal 

 to or greater than the body length (fig. 49, C). It is dis- 

 tinguished from stage 34 by the fact that the epitrichium 

 has not yet covered the eyes, ears, or mouth; and there are 

 no claws on the forefeet or digital ridges on the hind feet. 



The veins of the liver and the postcava. In stage 32 the 

 right subcardinal attained communication with the right 

 omphalomesenteric vein in the liver by way of the dorsal 

 mesentery and the right coronary appendage of the liver. 



Fig. 49 Photographs of external views of stage 33. A, ventral view of 16176. 

 B. lateral view of 16176. C, lateral view of 16177 showing maximal size of 



:illantois. 



This affords a very direct path for the blood from the pos- 

 terior part of the body to the right atrium, and in stage 33 

 this channel enlarges until it becomes the most conspicuous 

 vein in the body (fig. 50, postcava). The posterior portion 

 of the omphalomesenteric appears now as a tributary to it. 

 This infrahepatic portion of the omphalomesenteric receives 

 the subintestinal vein, and is the anlage of the hepatic portal ; 

 but as it opens directly into the postcava, it is not yet, strictly 

 speaking, a portal, taking portal to mean a venous channel 

 interrupted by capillaries. 



