150 EDWARD MCCRADY, JR. 



The subintestinal vein. Senior ( '25) was the first to ob- 

 serve that the subintestinal vessel in the opossum is a vein, 

 and he pointed out that in this respect the opossum is like 

 some of the lower vertebrates, and unlike the placental mam- 

 mals, in which the vessel is always an artery. Kimball ('28) 

 made a comprehensive study of the vessel, and concluded that 

 it is a vein in all chordates in which there is no allantoic 

 placenta, and that it acquires new connections and becomes 

 converted into an artery when an allantoic placenta is formed. 

 This seems well established by her extensive investigation. 



In early stages of the development of the allantois in the 

 opossum (e.g., stage 30, 16160, slide 4) two very small veins 

 on its dorsal wall drain caudally into the subintestinal vein 

 and thence forward into the omphalomesenteric. I shall refer 

 to these as the primitive allantoic veins to distinguish them 

 from the much larger and more important umbilical veins 

 which become the definitive veins of the allantois. As the 

 allantois is a diverticulum from the hind-gut, it is natural 

 that it should carry with it some of the latter 's circulatory 

 apparatus, but these vessels never become large enough to be 

 of much functional significance, and they disappear altogether 

 by stage 31. 



In stage 32 (16174, 16173) the subintestinal vein thus has 

 no connection with the allantois. Its posterior end is very 

 plexiform and communicates by a few small branches with 

 the postcardinals, but its anterior portion is still well de- 

 veloped (fig. 47, D) and drains forward into the omphalo- 

 mesenteric. This is the stage which both Senior and Kimball 

 studied. 



Changes in the branchial pouches. The third pair of 

 pouches become detached from the pharynx in early stage 32 

 (16174, fig. 47, G). At this time no migration has occurred, 

 so all the elements are readily identifiable. 



In late stage 32 (17127) all of the gill pouches are detached 

 and considerable shifting has occurred so that determination 

 of the origin of the different elements is impossible without 

 a large number of closely graded specimens. I have only three 



