THE EMBRYOLOGY OF THE OPOSSUM 141 



Both of these venous plexuses show signs of degeneration 

 in stage 31 (16163), but particularly the dorsomedial or more 

 cranial of the two. The channels are becoming sinusoidal or 

 sacculated, and the endothelium is breaking down. Peri- 

 venous spaces form in the mesenchyme immediately around 

 these veins and develop an independent mesenchymal en- 

 dothelial lining before the endothelium of the veins breaks 

 down (fig. 48). These perivenous spaces are lymphatic an- 

 lagen. They do not form in the mesenchyme surrounding the 

 functional or permanent vessels in the same region, but are 

 consistently found around all venules which are to be re- 

 placed by lymphatic channels. When the endothelium of the 

 vein thus surrounded breaks down, the blood is liberated into 

 the primitive lymphatics, which are thus at first hemophoric. 



At the same time that these changes are occurring in the 

 region dorsolateral to the confluence of the cardinal veins, 

 another set of changes is in process in the mesenchyme ven- 

 tral to the aorta in the lower cervical and upper thoracic 

 region. Here the mesenchyme is becoming distinctly vacuo- 

 lated as if the intercellular fluid is increasing. As the mesen- 

 chymal clefts enlarge, the cells lining the spaces assume en- 

 dothelial characteristics. At first the lymphatic anlagen thus 

 formed are isolated, but later they run together to form a 

 plexus which is continuous with other mesenchymal spaces of 

 perivascular origin like those which coalesce to form the 

 jugular sacs. 



The thoracic duct thus is derived from two sorts of anlagen 

 some which form altogether independently of veins, and 

 others which form around degenerating venules. In all cases, 

 however, the anlagen develop their own independent en- 

 dothelium from mesenchyme. 



Changes in the arterial system. Among the aortic arches 

 the principal flow of blood seems to be shifting progressively 

 caudad, so that now the fourth arch has become the largest 

 of the group (fig. 45). The fifth is longer and slenderer than 

 in stage 30, but it still has a larger caliber than it ever at- 

 tains in most mammals. The endocardial ridges of the conus 

 are causing a partition to be formed between the base of the 

 fifth arch and the ventral aorta, pushing the fifth forward with 

 the fourth. The sixth has enlarged until it is about the same 

 size as the ventral aorta, which latter correspondingly looks 



