36 F. W. THYNG 



Into the anterior part of the roof of the prhnitive oral cavity 

 open the large primitive choanae {Ch.pr.) of the olfactory vesicles, 

 separated from one another by a primitive nasal septmii. The 

 roof also presents on either side of the median line, a prominent 

 longitudinal ridge, the palate process {Pr.pl.). This process of 

 the right maxillary arch is clearly shown in plate 4 where a por- 

 tion of the tongue has been removed. It begins at the intermaxil- 

 lary process iPr.i.m.) and extends dorsally, lateral to the choana 

 {Ch.pr.) along the primitive oral cavity and the cephalic part of 

 the pharynx. Its free ventral border nearly reaches the floor of 

 the pharynx in the region of the alveolo-lingual groove. The roof 

 of the oral cavity between the palate processes thus forms a high 

 arch which receives the dorsum of the developing tongue. It is 

 evident that the primitive oral cavity now comprises a portion 

 which will be cut off later by the union of the palate processes 

 and nasal septum, to fonn in part the nasal cavities of the adult. 



The hypophysis (Hyp.), alluded to above, consists of a distal 

 spade-like portion, connected to the oral epithelium in the median 

 line by a slender stalk with reduced lumen. It is represented in 

 side view in plate 4, in median sagittal section in plate 1. Its 

 flattened body impinges upon the ventral surface of the infundib- 

 ulum, on either side of which it projects dorsally as a short, 

 blunt process. On its cephalic surface there is a distinct ridge 

 or fold, continuous with the anterior surface of the stalk. 



Pharynx. A left lateral view of the entodermal wall of the 

 pharynx is seen in plates 2 and 5. The interior, as it appears 

 in median sagittal section, is represented in plate 1. It is a 

 broad, dorso-ventrally compressed canal which narrows rapidly 

 in passing caudally to divide into trachea (Tr.) and oesophagus 

 (Oe.). The epithelium of the roof and floor of the pharynx meet 

 to form an external ridge, which extends from near the angle 

 of the mouth to the lateral border of the oesophagus. Corre- 

 sponding with the ridge there is an internal furrow. 



The tongue (plate 1) is a comparatively broad elevation of the 

 floor of the pharynx, composed of a large cephalic part {t') in- 

 timately fused with a smaller caudo-lateral division {t"), the root. 

 A surface view of the dorsum of the tongue would show the line of 



