STRUCTURE OF THE VERTEBRATES 



193 



dodermal tube the mesodermal smooth muscles and mesenteries 

 are added. The digestive tract in its completed form consists of 

 the endodermal lining (the epithelium) and the contracting and 

 strengthening tissues. 



The blastopore, formed at the time of gastrulation, is posterior 

 and forms the anus. The mouth later breaks through at the an- 

 terior end. The early embryonic tract is a straight tube opening 

 to the exterior at either end. There is an infolding of ectoderm at 

 both mouth and anus, with the result that the anal opening and 



Gill potwhes 



Foregut 



l^-Liver 



Fig. B 



Fig. 109. Development of the Digestive Tract in Man. (A) The early 

 embryo in sagittal section showing the elongation of the tract and its 

 continuity with the yolk sac. (B) A later stage, the tract dissected out. 



a part of the mouth cavity are lined with ectoderm. The latter 

 is of particular interest in the development of teeth, which are 

 covered with a layer of enamel secreted by ectodermal cells. 



The primitive tube becomes lengthened and modified into more 

 or less sharply defined regions. In the lower vertebrates the 

 mouth cavity, the pharynx and the esophagus are practically 

 continuous. There is more separation between the regions in the 

 reptiles, particularly the alligators and allied members of the 

 order; and in the mammals the three regions are clearly de- 

 limited. The stomach is developmentally a pouch-like enlarge- 



