Lymphatics in Small Intestine of the Pig. 117 



a serosal plexus. In Fig. 16, from an embryo 20 cm. long, the 

 serosal plexns was injected by introducing the needle directly into 

 the wall of the intestine. Sections of this specimen proved that the 

 lymphatics are in the serous covering of the bowel. 



The origin of this plexus is then similar to that of the plexuses in 

 the mucous membrane ; that is, it is by the growth and extension 

 of lymphatic vessels through the serosa, these vessels being derived 

 from the primary lymph ducts. The mode of origin is indicated in 

 embryos younger than 10 cm., in which short, blunt-ended lymph 



Fig. 17. — A lymphatic tree injected in the intestinal wall of a new-born pig 

 to show that the injection of a single mesenteric trunk may isolate one of the 

 units even after the plexus is complete. 



ducts can be seen extending a short distance into the serosa. Wliile 

 this seems to be the origin of the subperitoneal plexus, it is difficult 

 to say that another process docs not enter into its formation, that 

 is a growth outward of lymphatics from the primary plexus in the 

 submucosa. In embryos 10.3 cm. long, serial sections of the intestine 

 show a few lymphatic vessels extending from the primary plexus 

 through the circular muscle coat toward the serosa and from the 



