Lymphatics in Small Intestine of the Pig. 



101) 



of as a unit of structure. These primary lymph ducts can be followed 

 in the ascending series of embryos and appear as the large collecting 

 trunks, such as are seen for example in Fig. 14. 



The formation of simple lymphatic loops in the submucosa as 

 the primary event in the development of the lymphatics of the 

 intestine has been demonstrated by repeated injections in embryos 

 from 6 to 10 cm. long. It has been found constant that lymphatic 

 vessels first appear in the submucosa and that the vessels branch to 

 form loop-like structures. By means of serial sections, these loops 

 are found to be continuous with the lymphatics of the mesentery. 



lipi»««i*(^^ 



Fig. 9. — Loop of small intestine of an embryo pig 9 cm. long to show the 

 primary lymphatic loops in the submucosa. 



By the branching of these primary loops and the fusion of these 

 branches Avith those of adjacent loops, a primary lymphatic plexus 

 is formed in the submucosa. This is shown in Fig. 10, from a pig 

 10 cm. long. 



In cleared whole preparations and in serial sections, branches from 

 the primary lymph ducts are seen extending out into the submucosa 

 toward one another. In different preparations it is found that some 

 of the branches from neighboring trunks have fused, while others 

 have approached one another but have not fused. Cleared specimens 

 from embryos 8.5 to 11 cm. long show this plexus formation very 

 well. 



It seems clear, therefore, that a primary lymphatic plexus is 

 formed in the submucosa by the branching of the primary loops and 



