VEINS AND LYMPHATICS IN TRAGULUS 195 



With reference to the development of the systemic lymphatics 

 the material does not furnish conclusive evidence in all particu- 

 lars. The organization of the lymph sac, however, is identical 

 with that observed in the domestic cat by Huntington and 

 McClure ('10). The younger embryos present all the characteris- 

 tics of the primary venous stage, while the older specimens show 

 the several details of structure typical of the late veno-lymphatic, 

 pre-lymphatic and definite lymphatic stages. The systemic 

 lymphatics, at first, are entirely independent of the lymph sacs 

 and only secondarily acquire connections with them in completing 

 the thoracic duct formation. In the specimens examined they 

 showed no genetic relation to the veins, although their exact 

 histogenesis could not be determined on account of the limited 

 material. 



The Born method of reconstruction was employed with a 

 magnification of 100 diameters for purposes of topographical 

 study and 200 diameters for obtaining regional detail. All meas- 

 urements in the embryo were made from sections after fixation 

 upon the slide and, in most cases, computed from figures obtained 

 at a given magnification. None of the embryos was injected. 



Venous organization of the 5 mm. embryo {fig. 2) 



This embryo presents an imperfectly closed neural tube in 

 the entire spinal region. Its venous channels have a bilateral, 

 symmetrical arrangement, there being four sets of paired trunks, 

 all parallel with the long axis of the body. In addition to these 

 drainage lines there are two plexuses, one, the perimescnephroic 

 plexus surrounding the mesonephros, the other, the umbilico- 

 cardinal plexus which is still active in draining the territory of 

 the body wall into the umbilical and post-cardinal veins. 



1 . The umbilical veins {fig. 2, 6) . These are the largest chan- 

 nels present ; they are as yet equal in size and show no tendency 

 to lose their bilateral, symmetrical arrangement. No com- 

 munication has been established with the hepatic sinusoids, so 

 that the two umbilical veins are still independent vessels. They 

 are, to a considerable extent, concerned in drainage of the body 



