1 8 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



chord, gives rise to the lining (epithelium) of the digestive 

 canal (alimentary tract) and its appendages, and of the respira- 

 tory organs (gills and lungs). 



The first step in the differentiation of the alimentary struc- 

 tures is the formation of an outpocketing on the ventral side, the 



beginning of the liver 

 (Fig. 20, /). This oc- 

 curs some distance ia 

 front of the middle of 

 the body, and di\ides 

 the alimentary canal 

 into pre- and post- 

 hepatic portions. The 

 post-hepatic portion 

 gives rise to the intes- 

 tine and its various di- 

 visions, including the 

 pancreas ; from the pre- 

 hepatic region are developed the pharynx, with the respiratory 

 structures, the gullet, and the stomach. 



The Mouth. — Besides these entoderraal structures, the ali- 

 mentary tract, as usually considered, embraces as well the cavity 

 of the mouth, the lining of which is ectodermal in origin. The 

 mouth arises as an inpushing or involution of the ectoderm ■' at 

 the anterior end of the ventral surface of the body. The in- 

 pushing usually takes the form of a cup, the blind end of which 

 impinges directly upon the closed anterior end of the alimentary 

 canal proper, thus forming a double partition between the two 

 (Fig. 55). These two membranes, one ectodermal the other 

 entodermal in origin, fuse, and then an opening breaks through, 

 thus placing the whole in communication. From this ectodermal 

 oral invagination or stomodaeum are developed the lips, teeth, 

 tongue, and glands. 



The lips bound the opening of the mouth. In all the lower 

 vertebrates they are merely folds of epithelium, or, as in turtles 



Fig. 20. Longitudinal section of Aniblystoma 

 embryo. ^, hypophysis ; ht, heart, its endo- 

 thelial walls not shown ; i, infundibulum ; iti, 

 intestine; /, liver; vi, mesenchyme; n, noto- 

 chord ; /, pineal outgrowth ; pc, pericardial cavity. 



1 In some forms this inpushing is plainly a paired structure, a fact which adds no 

 little weight to the view which regards the vertebrate mouth as having arisen from the. 

 coalescence of a pair of gill slits. 



