994 



DEVELOPMENT OF THE ALIMENTARY CANAL. 



At a later period the bowel forms^the first loop (Fig. 388, //), being rotated on 

 itself at the situation of the intestinal umbilicus, so that the lower portion of the 

 bowel lying next to the knee-shaped flexure is turned upward, and the upper 

 portion is turned downward. From the lower limb of this loop there grow the 



FIG. 388. Development of the Intestine: v, 

 Stomach; o, insertion of the omphalomes- 

 enteric duct; t, small intestine; c, colon; 

 r, rectum. (Diagrammatic.) 



FIG. 389. Development of the Lungs: A, 

 Evagination of the lungs as double sacs; 

 k, mesoblastic layer; /, entoblastic layer; 

 m, stomach; s, esophagus. B, Further 

 ramification of the lungs: /, trachea; b, e, 

 bronchi; /, budding glandular vesicles. 



m 



coils of small intestine, constantly increasing in length (///, t) . From the upper 

 limb, which increases in length, the large intestine is formed in such a manner 

 that first the descending colon, then by elongation the transverse colon, and 

 finally also the ascending colon results. 



The intestinal canal gives rise through evagination to various glands. This 

 process is participated in by the cells of the hypoblast, which become the secretory 



cells of the glands, as 

 well as the splanchno- 

 pleure, which supplies 

 the 'limiting membrane 

 of the glands. These 

 diverticula are in order : 

 (i) The salivary glands, 

 which at first are solid, 

 but soon develop from 

 the oral cavity as intri- 

 cately ramifying glandu- 

 lar bodies. (2) The lungs, 

 which develop as two 

 separate hollow vesicles 

 (Fig. 389, A, /), which 

 subsequently take their 

 origin from a common 

 tubular evagination of 

 the esophagus. The up- 

 per portion of the united 

 tracheal tube becomes 

 the larynx. The epi- 

 glottis and the thyroid 

 cartilage are derived 



from the rudimentary 

 tongue. The two vesicles 

 grow according to the 

 type of a branching, 

 tubular gland, with hol- 

 low buds (B,f). In the 

 earliest stages of de- 

 velopment there exists no particular difference between the epithelium of the 

 bronchi and that of the primitive air-vesicles. The spleen and the suprarenal 

 bodies, however, do not develop in this manner. The first is formed as a fold of 



FIG. 390. Development of the Great Omentum. / and //: kg, 

 hepatogastric ligament; m, greater and n lesser curvature of the 

 stomach; 5 posterior and i anterior layer of the omentum; me, 

 mesocolon; c, colon. /// (in addition to the letters in / and II): 

 L, liver; /, small intestine; b, mesentery; p, pancreas; d, duodenum ; 

 r, rectum; N, great omentum. (Diagrammatic.) 



