116 



George Heuer. 



The plexus of these vessels is wider meshed, and shows clearly the 

 primitive units, in contrast with the finer meshed mucosal plexus 

 of capillaries. This mucosal plexus gives rise to the central lacteals, 

 as is seen in both Figs. 14 and 15. 



The development of the third lymphatic plexus of the intestine 

 of the pig, namely that confined to the muscularis and serosa, has 

 been the most difficult to study. It apparently develops later than 

 the plexuses in the mucous membrane and an injection of what could 

 be definitely termed a plexus has not been obtained in embr^'os under 

 10 cm. in length. In embryos up to 9 cm. in length no injected lym- 



FiG. 16. — Injection of the serosal plexus in a loop of intestine from a pig 

 20 cm. long. 



phatics have been found between the muscle coats ; and it appears 

 that the lymphatics in these younger stages are developed only in 

 the mucous membrane. 



In following the lymphatics through the wall of the intestine, 

 it was found, as previously stated that the lymph ducts may pass 

 in the serum some distance around the wall of the intestine before 

 penetrating the muscle layers. Branches may be given off from the 

 lymph ducts, while lying in this layer and by the intercommunica- 

 tion of these branches the beginning of a subperitoneal plexus may 

 be formed. Such vessels have been found as early as 10 cm. In 

 older embryos, as for example at 20 cm., it is easy to demonstrate 



