118 George Heuer. 



series we can feel fairly certain that these are not vessels coming 

 into the intestine. In embryos 16 cm. long, such branches are more 

 numerous and can be traced for some distance between the muscle 

 layers before penetrating the longitudinal coat, to enter the serosa. 

 Such vessels extend into the serosa and may help to form the plexus 

 there. They form the connecting vessels between the plexuses in 

 the mucous membrane and the serosa. 



From the preceding study it may be seen that the evolution of 

 the lymphatic system of the intestine is from the center to the 

 periphery. The retroperitoneal sac is the origin of the lymph vessels 

 of the intestine; by repeated injections it has been shown that this 

 sac is the best place from which to inject the intestinal vessels, and 

 that corresponding with the growth of the embryo an increasing 

 zone of lymphatics can be injected. From the sac vessels grow out 

 into the mesentery making a plexus which extends to the wall of 

 the gut. The primary mesenteric lymphatics enter the mesentery 

 with the artery. From the mesenteric plexus a series of vessels 

 enters the wall of the intestine with the branches of the mesenteric 

 artery. These lymphatic vessels penetrate to the submucosa and 

 form there a series of loops extending around the wall of the gut. 

 These loops represent lymphatic units, which soon become united 

 into a complete but coarse-meshed plexus. From this submuosal 

 plexus, the mucosal plexus of smaller vessels develops. The mucosal 

 plexus is fine meshed. From the mucosal plexus, the lacteals grow 

 into the villi. The serosal plexus develops late from the lymphatic 

 trunks as they are entering the bowel wall. The retroperitoneal sac, 

 which is the origin of the lymphatic vessels of the intestine, is the 

 anla,ge of the retroperitoneal, preaortic chain of lymph nodes. The 

 first evidences of the formation of the nodes from the sac occur as 

 early as 3 cm., when the sac begins to be bridged by connective tissue 

 bands. The preaortic nodes are all primary ones for the intestine ; 

 the mesenteric nodes develop along the course of the lymphatic 

 ducts and form the secondary group. Thus, the primary nodes, are 

 those that come from the primitive sacs, secondary and tertiary 

 nodes, etc., develop along the course of the vessels. 



