Lymphatics in Small Intestine of the Pig. 105 



vessels is marked at 8 cm., as is shown in Fig. 7. During the 

 stages from 4 to 10 cm. long, a number of changes take 

 place, in the first place the plexus in the mesentery becomes exceed- 

 ingly complex. For example, at 8.5 cm., an injection directly into 

 the retroperitoneal sac will fill the entire mesentery with an abundant 

 plexus of ducts; moreover, at the root of the mesentary, along its 

 entire length, the injection mass fills in so as to appear solid to the 

 naked eye. This long line of exceedingly abundant injection cor- 

 responds with the long line of single mesenteric nodes which are 

 characteristic of the pig. Thus both the primary, that is retroperi- 

 toneal, and the secondary or mesenteric lymph nodes are forming at 

 this stage. 



To return to the wall of the intestine, it is shown in Fig. 4 that 

 lymphatic vessels have reached the stomach and intestine in an 

 embryo 4.5 cm. long. Injections made directly into the sac show 

 that the vessels reach the intestine in embryos about 4 cm. long. 

 The injections shown in Figs. 8, 11 and 12 were all made into the 

 thoracic duct before it was found that puncturing the sac gives 

 better results ; and some of them are incomplete, but they serve 

 to illustrate the progression of the lymphatics. In Fig. 4, it is seen 

 that the lymphatics form a plexus in the mesentery and from this 

 plexus a series of lymph vessels grows into the intestine along the 

 arteries. These vessels enter the submucosa and form there a 

 primary plexus. This primary submucosal plexus, at a stage when 

 there is no secondary plexus, is shown for the stomach for a pig 

 6 cm. long. The drawing is not made so that it can be oriented 

 readily, but the heavy mass is at the lesser curvature, and the plexus 

 shown is in the submucosa. 



In Fig. 8, is shown a section of the duodenum of a pig 8.7 cm. long. 

 By this time there is not only a submucosal plexus but a secondary 

 mucosal plexus as well. As has been said, the lymphatics which 

 grow into the intestine at the mesenteric border penetrate the longi- 

 tudinal and circular muscle coats, and enter the embryonic submucosa. 

 In following through a large number of series, no deviation from 

 this course has been observed. The point at which the lymph ducts 

 penetrate the intestine, however, is subject to variation within certain 



