148 EDWARD MCCRADY, JR. 



munication with the venous system at all. The endothelium 

 of the lymphatics is of mesenchymal origin and is not derived 

 from, or until a later stage even connected with, the venous 

 endothelium. 



Heuser's note ('19) about the anatomy of a specimen of 

 this stage referred to the jugular lymph sac as having 

 "become transformed into a much-divided structure with 

 smaller and larger spaces." Instead of the sac having be- 

 come divided into the several elements referred to, the latter 

 are really in process of uniting to form the sac. His further 

 note that "In an injected embryo of the same litter the lymph 

 sac received no ink, although the injection of the blood vessels 

 is complete" is in harmony with Zimmermann's observations 

 that the jugular lymph sac has not yet acquired communication 

 with the venous system. 



Though the lymphatic system has only begun to form at 

 this time, the method of origin thus far described will apply 

 equally well' to the later stages. In far the majority of cases 

 the original lymphatic anlagen are spaces which form around 

 collapsing venules. Such a peri-intimal space acquires an 

 endothelium of its own (fig. 48) before that of the vein has 

 disintegrated. When the venous endothelium breaks down, 

 blood is liberated into the lymphatic which is thus for a while 

 hemophoiic. The separate lymphatic anlagen coalesce to 

 form a plexus which secondarily acquires communication with 

 the venous system. 



The fact that the lymphatics usually, though not invariably, 

 form around collapsing venules, accounts for the frequently 

 recorded observation that they nearly always follow the path 

 of a pre-existing venous plexus. In all cases they are of 

 mesenchymal origin, and they never derive their endothelium 

 from veins. 



The adrenal cortex. In late specimens of stage 31 and in 

 all specimens of stage 32 cords of mesodermal cells proliferate 

 from the peritoneum near the mesentery at the level of the 

 posterior end of the lungs and caudad as far as the region 

 of the stomach. These cords of cells, the anlage of the cortical 



