98 George Heuer. 



still beating. For any stage it is easier to obtain complete injections 

 through the sac than through the thoracic duct, for in the later case 

 the injection mass must fill the sac before it runs out into the vessels, 

 and the size of the sac decreases the pressure. 



Figs. 1 to 4 and Y show that the thoracic duct is fairly sym- 

 metrical below the heart, that there are two vessels, one on either 

 side connected by many cross channels making a plexus around the 

 aorta. Dorsal to the heart, the right duct crosses over and joins the 

 left. Dorsal to the kidneys, the two ducts unite in a median cisterna 

 chyli. 



In embryos above 12 cm. it was found impossible to obtain in- 

 jections of the intestinal wall through the thoracic duct. This is 

 due to the developing lymph nodes, which at first do not check the 

 flow of the injection mass, but later retard it very much. It has 

 been shown that in human embryos the retroperitoneal sac is changed 

 into the group of preaortic lymph nodes extending from the coeliac 

 axis to the bifurcation of the aorta, and this change is being made 

 in an embryo 8 cm. long. A secondary, larger group of lymph nodes 

 forms in the center of the mesentery, along the superior mesenteric 

 artery, and this group is also being formed in the same embryo. 

 The early lymph nodes, however, consist of a great plexus of wide 

 lymphatic ducts with very few follicles and hence injection through 

 them is easy. By the time the pig is 12 cm., however, the injection 

 mass passes to the nodes, and increased pressure results in an 

 extravasation at the node. For these stages, therefore, it was found 

 necessary to inject into the wall of the intestine itself. To get 

 good injections by this method it was found best to pierce both 

 muscle coats, and thus have the needle just enter the sub-mucosa 

 and inject slowly under low pressure. In embryos from 12 to 13 

 cm. this was difficult, in the older stages, from 16 cm. up, it could 

 be done readily. 



The areas injected by these two methods, in the younger embryos 

 through the thoracic duct or retroperitoneal sac, in the older pigs 

 directly into the wall of the intestine, were of course very different. 

 By injecting into the sac, a general lymphatic injection was obtained, 

 the lymphatics reaching in embryos 5 or 6 cm. long all the thoracic- 



