336 PROFESSORS ARTHUR ROBINSON AND A. GIBSON. 



The Alimentary Canal. 

 The alimentary canal comprises an ectodermal stomatodseum, an entodermal 

 fore-gut, and a partially enclosed hind-gut. The mid-gut is not yet separated from 

 the yolk-sac. 



The Stomatodseum. 



The stomatodseum is best seen from the ventral aspect. It is a relatively wide and 

 deep recess overhung on the cranial side by the massive fronto-nasal process, and 

 bounded laterally and caudally by the mandibular arches (figs. 56, 57, 58, PI. XIX). 



Maxillary processes of the mandibular arches and Kathke's pouch are not yet 

 formed. 



Bucco-pharyngeal Membrane. 



The bucco-pharyngeal membrane is still present. It consists of ectoderm and 

 entoderm alone. Dorsally it is attached to the ventral surface of the head, where 

 its entodermal layer is fused with the cranial end of the notochord. Laterally and 

 ventrally it is connected with the cranial borders of the mandibular arches (fig. 57, 

 PI. XIX). 



SeesseVs Pouch. 



Immediately caudal to the dorsal end of the bucco-pharyngeal membrane there 

 is a small dorsal diverticulum of the cranial end of the fore-gut. It obviously corre- 

 sponds in position to Seessel's pouch (fig. 57, PI. XIX). 



The Fore-gut. 



The dorsal wall of the fore-gut follows the contour of the ventral walls of the 

 mid- and hind-brains (fig. 57, PI. XIX). 



In the ventral wall of the fore-gut, immediately caudal to the fused ventral ends 

 of the mandibular arches, there is an eminence of triangular outline. It is formed 

 by a mass of entodermal epithelium which lies immediately dorsal to the cranial end 

 of the truncus aorticus, and it probably represents the tuberculum impar. Along 

 each cranio-lateral border of the tuberculum impar there is a slight ridge on the 

 ventral end of the corresponding mandibular arch ; therefore at this stage there are 

 present, already, three rudiments of the tongue (fig. 63, PI XIX). 



The median part of the caudal border of the tuberculum impar is connected, by 

 a median ridge, with a transverse elevation which unites the ventral ends of the 

 second branchial arches across the ventral wall of the pharynx (fig. 63, PI. XIX). 



Branchial Pouches. 



Four branchial pouches are present (figs. 57, 58, PI. XIX). The first and second 

 are relatively voluminous. They communicate with the cavity of the pharynx by 

 wide apertures, which are situated immediately ventral to the dorsal aorta, and 



