Lymphatics in Small Intestine of the Pig. 



Ill 



like the preceding, is the result of the peripheral spread of the lym- 

 phatics through the intestine, a plexus being formed by branching 

 and the fusion of neighboring branches. The beginning formation 

 of this second plexus is seen in stages in which the primary plexus 

 is but incompletely formed and, therefore, we must think of the 

 two plexuses as developing more or less simultaneously. In embryos 

 8.5 to 9.0 cm. long, in which primary lymph ducts and an incom- 

 plete submucosal plexus are found, branches are seen coming off 

 from the primary plexus, these branches extending toward the villi. 

 These branches mark the beginning development of the mucosal 



Fig. 11. — Section of the duodeuum of a pig S.7 cm. long to show the sub- 

 mucosal plexus, the mucosal plexas and the beginning of the lacteals. 1., lac- 

 teals ; m. p., mucosal plexus ; s. p., submucosal plexus. 



plexus. They have been found in embryos 8.7 cm. long, as seen 

 in Fig. 8, and still better in Fig. 11. Fig. 11 is from a section 100 

 microns thick and not all the lymph vessels lie in the same plane, as 

 shown in the drawing. It will be seen that the primary plexus is repre- 

 sented by an almost complete loop lying nearest to the circular muscle 

 coat. From it branches have been given off extending inward toward 

 the villi. In the older embryos and in the new-born and adult pigs, 

 these branches remain as the connecting vessels between the mucosal 

 and submucosal plexuses. In two places in the section, these 

 branches have been themselves branched to form lymph-vessels 

 running along at the base of the villi. This is an important step 



