THE EMBRYOLOGY OF THE OPOSSUM 79 



the chick the myocardial elements which approach each other 

 near the midline are only half cylinders (B). As a result, 

 when they fuse, mesenteries are formed both dorsal and ven- 

 tral to the heart tube (C and D). In the opossum, on the 

 other hand, each hemicardium is a nearly complete tube (E) 

 long before the two approach each other. When they do draw 

 near the midline (F, G, H, I) each is suspended by a single 

 dorsal mesocardium. The two dorsal mesocardia fuse when 

 the tubes meet (J and K), and no ventral mesocardium is 

 formed. 



Origin of the sinus venosus. In the last chapter it was 

 mentioned that the allantoic vein arises just dorsad to the 

 point where the vitelline vein enters the body. The two 

 vessels are at first separated by a slight interval of coelom, 

 but as they enlarge, the parietal layer of pericardium which 

 underlies the allantoic vein, meets and fuses with the visceral 

 layer which overlies the vitelline vein, and the vessels thus 

 come into contact with one another (fig. 25, H). In late 

 specimens of stage 25 their walls break down at the point 

 of contact and they become confluent. Hereafter the original 

 vitelline vein anterior to the point at which it receives the 

 allantoic is the sinus venosus. 



At the same time that this communication is established, 

 capillary sprouts growing caudad from the vena capitis medi- 

 alis, or primitive precardinal, reach this same level, and a 

 few fuse with the allantoic vein near its entry into the sinus. 

 The head vein thus gains access to the sinus. Some of the 

 growing tips, however, pass this level (which is opposite the 

 second somite) and continue caudad to the neighborhood of 

 the fourth somite. These constitute the anlage of the post- 

 cardinal vein, which, however, will not be clearly established 

 until stage 26. 



Origin of the pliaryngeal floor. In the early specimens of 

 stage 24, though two branchial pouches with closing plates 

 had formed, there was no pharyngeal floor beneath them. 

 Figure 20, section B and reconstructions L and J, shows the 

 small thimble-shaped diverticulum which is the anterior ex- 

 tremity and first-formed portion of the foregut. The same 



