DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 345 



the lymphoid cells dividing actively, and grow into the bases of some of the 

 villi and deeper into the submucosa (Fig. 309). Germinal centers appear in 

 many of the follicles, and the surrounding stroma becomes densely infiltrated 

 with the lymphoid cells. Individual follicles may develop, in the manner 

 described, in any part of the gastrointestinal tract. The appendix especially is 

 the seat of extensive lymphatic tissue formation. It is stated in the section on 

 the lymphatic system that lymph glands may arise at any time in any region as 

 the result of unusual conditions (p. 282), and this also holds true in the case of 

 lymph follicles in the digestive tract. 



The Development of the Liver. 



The liver is the first gland of the digestive tract to appear. In embryos of 

 about 3 mm. a longitudinal ridge-like evagination develops from the entoderm 

 on the ventral side of the gut a short distance caudal to the stomach, that is, in 



Myotome 

 Aorta 



Post, cardinal vein 

 Coelom 



Upper limb bud 

 Dorsal mesentery 



Duodenum 

 Liver 



Omphalomesenteric vein 

 Umbilical vein 



Heart 



FIG. 310. Transverse section of a human embryo of 5 mm., showing the liver evagination and the 

 breaking up of the omphalomesenteric veins by the hepatic cylinders. Photograph. 



the duodenal portion of the gut (Figs. 285, 310, 311). The cephalic part of the 

 evagination is solid and, being destined to give rise to the liver proper, is called the 

 pars hepatica. The caudal part is hollow, its cavity being continuous with the 

 lumen of the gut, and is destined to give rise to the gall bladder, whence it is 

 called the pars cystica. Beginning at both the cephalic and caudal ends, the 

 evagination as a whole becomes constricted from the gut until (in embryos of 

 about 8 mm.) its only connection with the latter is a narrow cord of cells which 



