The Origin and Development of the Lymphatic System. 47 



In tracing the lymphatics cerebralward, from the level of the hilus 

 of the kidney, there is a small lymphatic capillary plexus in the place 

 of the cisterna chyli and the two lateral sacs become a plexus of small 

 ducts which can be traced the entire length of the permanent kidneys 

 and then along the dorso-lateraJ. surface of the aorta, jus! ventral to the 

 azygos veins. This small plexus of ducts is nearly empty on the left 

 side, but full of blood on the righl side. The two plexuses can be 

 followed almost to the level of the bifurcation of the trachea. Some 

 blood in the plexus on the left side just before the vessels end makes 

 it possible that the renal part of the thoracic duct does not yet connect 

 with the jugular part of the duct. 



This particular specimen is interesting in regard to certain /.ones 

 of dilated tissue spaces which arc conspicuous along the aorta. There 

 are some of these large spaces near the bifurcation of the aorta, in 

 the root of the mesentery near the retroperitoneal sac, and along the 

 ventral surface of the aorta, especially opposite the mesentery of the 

 stomach and opposite the bifurcation of the trachea. I have studied 

 them with care to see if they have any definite relation to the develop- 

 ing lymphatics and am sure that they have not. Sometimes they are 

 near the lymphatics and sometimes not. Those in the thorax are no! 

 adjacent to the lymphatic plexus. They can be distinguished from 

 the endothelial-lined lymphatics, but if one were convinced that lym- 

 phatics came from tissue spaces one might imagine transition pictures 

 between the tissue spaces and the ducts. Tins is particularly true when 

 the endothelium of a lymphatic vessel sags away from the surrounding 

 tissue, a picture familiar to histologists in sections showing lymphatics 

 in adult tissues. The further discussion of the thoracic duct is in 

 part VIII. 



All of the primary lymphatic system is present in a human embryo 

 measuring 30 mm. ( Mall collection No. 86, lig. 12, Sabin 133, copied as 

 tig. 493, Strieker 144). This stage represents the maximum size of the 

 jugular sac. The dorsal arch is huge and its line of separation from 

 the jugular part is indicated by the perforation for the cervical nerves 

 and blood vessels. The very small mesenteric sac and larger iliac sacs 

 are distinct as well as the complete thoracic duct. 



.'!. Primary Lymphatic System in Rabbit Embryos. 



In 1906 F. T. Lewis (7(!) published a valuable paper on the lym- 

 phatic- system in rabbit embryos. Besides excellent reconstruction of 



