158 OUTLINES OF CHORDATE DEVELOPMENT 



III. THE ALIMENTARY TRACT AND ITS APPENDAGES 



In the preceding chapter we described the formation of the 

 enteron or gut cavity and its development up to the time the 

 embryo is just beginning to elongate (Fig. 37). We have de- 

 scribed, therefore, the formation of the fore-. mid- f and hind- 

 jut and have seen that up to this time the chief differen- 

 tiations are connected with the fore-gut. After the mesoderm 

 and the notochord have been split off from the endoderm^the 

 wall of the enteron is but one cell layer in thickness, excepting 

 the floor of the mid-gut which is occupied by the large yolk- 

 mass. The hind- and mid-gut cavities are narrow, while the 

 fore-gut expands widely in front of the yolk-mass, and in con- 

 nection with it we have described the first indications of the 

 mouth region, of two or three visceral pouches, and of the 

 liver. 



We may proceed now to outline the further development 

 of each section of the alimentary tract. Before taking up the 

 history of the fore-gut we must notice the development of 

 certain structures associated with the mouth. 



The stomodceum has already been mentioned as a shallow 

 median depression of the head, just below the olfactory and 

 fore-brain region. At hatching this is still rather shallow, but 

 its floor has come into contact with the wall of the enteron, 

 establishing a fusion know r n as thejora/ plate (Figs. 57, 58, A), 

 for this is the region where the mouth forms a few days after 

 hatching (9-10 mm.). In the older larvre the inner bound- 

 ary of the stomodseal region is marked by the internal napes or 

 choanaB, which lie just within the boundary between ecto- 

 dermal and endodermal territory. The formation of the oral 

 sucker, just below the stomodseal invagination (Fig. 57), was 

 described in the previous chapter. 



The margins of the stomodaBum are at first formed chiefly by 

 the mandibular ridges, but soon the integument above these 

 becomes drawn out in the form of lips. The mouth itself 

 remains small in the tadpole, but the lips, of which there are 

 an upper and a lower, soon project considerably in front of 



