632 THE DIGESTIVE SYSTEM 



prominent modifications of the internal lining or mucous membrane tend to 

 occur: 



(a) Small finger-like projections or villi are formed which project inwardly 

 into the lumen (fig. 298A); and 



(b) the internal lining may project inwardly in the form of extensive elon- 

 gated folds. 



In many fishes, such as the sharks, lungfishes, ganoids, and cyclostomes, 

 elaborate folds of the mucosa, known as the spiral folds or valves, are formed 

 (fig. 291C). Similarly, in higher vertebrates, elongated folds may occur, such 

 as the valves of Kerkring in the human and pig small intestine (fig. 298B). 



Another conspicuous feature of the early histogenesis of the entodermal 

 layer is the formation of epithelial membranes and plugs. The pharyngeal 

 membrane is formed by the stomodaeal ectoderm and pharyngeal epithelial 

 layers. The proctodaeal membrane is similarly constructed. This structure 

 serves as a temporary blocking device between external and internal media. 

 Under normal conditions these membranes degenerate and disappear, although 

 occasionally they may persist. Epithelial plugs, temporarily obUterating the 

 lumen of the digestive tract, appear with regularity in many vertebrates. Such 

 temporary obstruction, for example, may appear in the developing digestive 

 tract of the chick or in the human esophagus, duodenum, and other areas of 

 the digestive tract. 



/. Differentiation of the Cloaca 



As previously observed, the caudal end of the intestine expands into the 

 cloaca, an enlarged area which eventually receives the urinary products as well 

 as the intestinal substances. The differentiation of this area is considered in 

 Chapter 18. 



C. Physiological Aspects of the Developing Gut Tube 



Within the developing digestive tubes of the shark, reptiles, birds, and 

 mammals, a brownish-green, pigmented material appears during the latter 

 phases of embryonic development. This material is composed of cells, bile 

 pigments, mucus, etc. It is discharged during the period just before or after 

 parturition. Fetal swallowing of ammionic fluid, gastrointestinal motility, the 

 pfesence of enzymes, fetal digestion and absorption, and defecation are well- 

 established facts in the physiology of the developing digestive tract of the 

 mammalian fetus (Windle, '40, Chap. VII). 



