112 George Heiier. 



in the development of the plexus in the mucosa, for it shows the 

 way in which this plexus is formed. In embryos 9, 10, and 11 cm. 

 long, the branching of the vessels has increased and numerous 

 branches have united to form a close-meshed plexus at the bases 

 of the villi. The vessels of this mucosal plexus are of smaller size 

 than those of the preceding plexus and remain so through the 

 ascending series. The plexus remains a close-meshed one throughout, 

 which was seen not to be the case in the submucosal plexus. 



The central lymph vessels of the villi of the intestine develop from 

 and almost simultaneously with the secondary lymphatic plexus of 



Fig. 12. — Composite section made from four adjacent sections to sliow the 

 central villus, from the small intestines of a pig embryo 9 cm. long. 



the mucosa. From a study of injections, they, like the plexuses 

 of which we have spoken, seem to be the result of the peripheral 

 extension of the lymphatics throughout the wall of the intestine, 

 the villi being the peripheral limit of this extension. Repeated 

 injections in the younger embryos have shown no lymph vessels in 

 the villi. In an embryo 8.7 cm. long, as we have seen, a beginning 

 plexus in the mucosa has been formed and in following through this 

 series a few short branches have been found which have extended 

 into the bases of the villi. In embryo 9 cm. long, the central lymph 

 vessels of the villi have been injected, and they appear as straight 



