Lymphatics in Small Intestine of the rPig. 103 



vessels, a sciatic group, a femoral and an umbilical. It is clear 

 then that in the embryo, lymph from the legs has the direct course 

 through the posterior sac to the cisterna chyli, or the indirect path 

 through the posterior sac and retroperitoneal sac. This, in connection 

 with the fact that the abdominal viscera, the diaphragm and the 

 lymphatics of the lungs are most readily injected from the retro- 

 peritoneal sac, is of importance in emphasizing the significance of 

 the retroperitoneal sac and the preaortic group of nodes into which 

 it develops. In an embryo 5.5 cm. long a single puncture into the 

 retroperitoneal sac injected the lymphatics of the abdominal 

 viscera, the skin of both hips and legs, the diaphragm, lungs, 

 esophagus and lymphatics of the skin of the head. Such very ex- 

 tensive anastomosis of all the lymphatic vessels of the embryo is 

 of significance as a basis for variations in the adult. 



To sum up the lymphatics at this stage, namely at 4.5 cm., the pri- 

 mary system is complete, that is the primary lymph sacs are formed 

 and connected into a system by the thoracic duct. Above the diaphragm 

 the vessels have reached the heart, and the esophagus, probably the 

 lungs also. Below the diaphragm vessels from the anterior part 

 of the sac have reached the spleen, the stomach, the intestinal wall, 

 the kidneys, suprarenal bodies and Wolffian bodies. There is an 

 anastomosis with the posterior lymph sac. From the posterior lymph 

 sac vessels follow the sciatic, femoral and umbilical veins. 



For the small intestine the lymphatics extend along the superior 

 mesenteric artery. In human embryos it has been shown that the 

 mesenteric sac spreads along the suprarenal veins to the root of 

 the superior mesenteric artery. In the study of the growth of lym- 

 phatic capillaries, it proves that these delicate walled vessels grow 

 along some thicker walled vessel ; the earliest lymphatics grow along 

 the veins, but in the case of some of the viscera other vessels or ducts 

 may be followed, as for example the mesenteric arteries or the bronchi 

 in the lungs. The border zone of the injected lymphatic capillaries 

 is marked by the rounded blunt ends which are characteristic of 

 injections of terminal lymphatics. In studying the border zones 

 of growing lymphatics it has been shown that the growing tips are 

 either smooth and rounded or have long slender endothelial sprouts 



