108 George Heiier. 



root of the mesentery that these vessels followed the artery in their 

 development. It is equally true that the lymphatic ducts follow the 

 arteries through the wall of the intestine. This can be seen in serial 

 sections and in whole specimens which have been cleared by the 

 method of Schultze. In serial sections of injected material, one 

 can readily find a section showing an artery passing through the 

 wall of the intestine ; and in such a section often one and sometimes 

 two lymphatic vessels are seen closely accompanying the artery 

 through the bowel wall. 



The arteries in their course to the intestine show the variations 

 which we have described for the lymphatics ; that is, the artery 

 either penetrates the wall of the intestine immediately on reaching 

 it, or passes some distance under the serosa before doing so. Since 

 the lymphatic vessels follow the arteries to the intestine, the variation 

 in this course is accounted for by the distribution of the blood 

 vessels. 



To return to Fig. 8, from an embryo 8.7 cm., beside showing the 

 beginning of the mesenteric lymph nodes and the method of the en- 

 trance of the lymphatics into the wall of the gut, the section also 

 shows that the submucosal plexus has spread entirely around the 

 wall of the intestine. 



On entering the submucosa, the lymphatic trunk divides into 

 two branches, one of which extends around either side of the wall 

 of the intestine. In its course, each branch lies entirely in the sub- 

 mucosa. In Fig. 9 is shown an important point in connection 

 with these branches of the intestinal wall. It is from a pig 9 cm. 

 long, and shows that these primary branches form a series of more 

 or less complete lymphatic loops lying in the submucosa, near the 

 circular muscle coat. This is, probably, an incomplete injection at 

 this stage, but it serves to illustrate the point, for in making the 

 injections the primary vessels of an area the vessels that develop 

 first, fill first, and those that develop later inject later. Thus a vas- 

 cular unit can be shown by a partial injection as seen in Fig. 17 for 

 the new bom pig. This can be observed even in the adult. 



The growth and the arrangement of these vessels indicate a seg- 

 mental development, and each with its future branches may be thought 



