DEVELOPMENT OF THE ALIMENTARY TUBE AND APPENDED ORGANS. 343 



ment there are two factors to be considered: (i) The formation of folds to in- 

 crease the absorbing surface and (2) the formation of secreting organs or glands. 

 As to the relation between these two factors there is a difference of opinion. 

 Some hold that both kinds of structures are the result of the same formative 

 process, that is, that the glands are simply the depressions or pits formed by the 

 intersection of folds at various angles, and that the folds are produced primarily 

 by the growth of the epithelium and mesenchymal tissue into the lumen of the 

 gut. Others maintain that although the folds may be produced by the growth 

 of the epithelium and mesenchymal tissue into the lumen, the glands arise as 

 independent growths of the epithelium into the subjacent tissue. The latter 



view is supported by the fact that in 

 some Amphibia the glands appear before 

 the folds (Fig. 307). Recent work on 

 Mammals also favors this view. 



The development of the folds and 

 glands begins in the different parts of the 

 gastrointestinal tract at different times. 

 It begins first in the stomach, then in the 



FIG. 307. Section through the wall of the . . . . . . . 



stomach of a frog embryo. Ep., Epi- duodenum, then in the colon, and then 

 theiium, with glands; sfo. submucosa; j the jejunum whence it progresses 



Muse., muscle layer. Ratner. J J 



slowly into the ileum. In the stomach 



it is uncertain whether the crypts and glands are depressions left among 

 projections of the mucous membrane, or the glands represent evaginations of 

 the epithelium into the underlying tissue. In the case of the large intestine 

 the same uncertainty exists. If the so-called glands are depressions among 

 villous projections that grow into the lumen of the intestine, they are not true 

 glands from an embryological point of view. 



Studies of the development of the mill in the human small intestine have led 

 to the conclusion that they are formed primarily as growths of the mucosa into 

 the lumen. In embryos of 19 mm. the mucosa of the cephalic end is thrown 

 into a number of longitudinal folds (Fig. 308). These then develop pro- 

 gressively toward the caudal end. Beginning in embryos of 50 to 60 mm. the 

 longitudinal folds become broken transversely into conical structures, the 

 villi. The intestinal crypts (of Lieberkuhn) possibly represent outgrowths of 

 the epithelium from the bottoms of the intervillous spaces. The duodenal 

 (Brunner's) glands are possibly to be considered as a continuation of the pyloric 

 glands of the stomach. They apparently grow as evaginations from the 

 intervillous crypts. 



The epithelial lining of the gastrointestinal tract is from the beginning a 

 single layer of cells, although the individual cells are altered in shape and 

 structure and acquire different functions in different regions. There is still 



