THE EMBRYOLOGY OF THE OPOSSUM 171 



The venules around which the arms of the Y-shaped 

 thoracic duct form are tributaries of the common cardinal and 

 precardinal veins principally, with a few in the more pos- 

 terior levels from the supracardinals. 



The thoracic duct proper, exclusive of the cisterna, is thus 

 seen to be derived from three sources: 1) medial outgrowths 

 from the paired jugular lymph sacs; 2) mesenchymal vacu- 

 oles in the mediastinum not associated with venous channels 

 3) peri-intimal spaces around degenerating mediastinal ven- 

 ules. The venules referred to are tributaries of the post- 

 cardinal, supracardinal, common cardinal, and precardinal 

 veins. The postcardinal and supracardinal contribute the 

 plexus around which the anlage of the single median part of 

 the duct forms. The common cardinal and precardinal veins 

 supply the plexus around which the bilateral arches connecting 

 with the outgrowths from the jugular sacs develop. 



In the same way, the external mammary lymphatics, which 

 also arise independently from perivenous spaces, attain com- 

 munication with the intermediate caudal jugular extension. 

 Further caudad the external mammary lymphatics develop 

 connections with the deep axillary portion of the jugular 

 lymph sacs, and sometimes with the subclavian vein near its 

 origin. 



Miscellaneous details. The chorioid plexus is beginning to 

 form in the fourth ventricle. The cerebellar thickening ex- 

 tends across the midline (Larsell, '35, figs. 20 to 37). The 

 pontine flexure is sufficiently far advanced to cause the head 

 to lift away from the chest. 



Semilunar valves have formed in the aorta and in the pul- 

 monary trunk. Mesenteric branches of the omphalomesen- 

 teric veins have become the principal channels, as the vitel- 

 line plexus is showing retrogressive changes. The allantoic 

 vessels are also becoming reduced in caliber. 



The lateral semicircular canal of the inner ear is formed. 

 Jacobson's organ elongates. 



The tail contains about 22 somites. 



