158 ALIMENTARY CANAL 



faucium, so that the primitive mouth or stomodoeum does not by any means correspond with the 

 permanent mouth. In fact, the floor of the mouth, including the tongue, is developed behind 

 the septum, and therefore in connexion with the fore-gut rather than with the stomodoeum ; 

 whereas the uppermost part of the pharynx, including the choanse, is in front of the septum, 

 and therefore belongs to the stomodceum. 



After the several prominences have united with one another, as described on 

 p. 86, to form the face, the primitive mouth is converted into a transversely 

 disposed cleft, and is divided by the development of the palate into an upper 

 nasal and a lower buccal portion (see p. 154). On the margins of the processus 

 globulares and maxillary processes where they form the upper, and also on the 

 mandibular arches where they form the lower border of the mouth, shallow grooves 

 running parallel to their outer edges appear. These are due to the presence of 

 ingrowths of the epithelium (Kollman), which are divided by a shedding of the 

 central cells into two lamellae. The fissures thus produced gradually deepen anc 

 separate off the lips from the edges of the developing jaws on which the denl 

 ridges are being developed. The vestibulum oris is produced by an extension oi 

 the clefts between the cheeks and the alveolar edges of the jaws. The cheel 

 themselves are formed by a union of the primitive lips as the buccal opening is 

 gradually constricted. 



PHARYNX. 



The anterior extremity of the fore-gut becomes dilated to form the pharynx. 

 Its cavity is greatly flattened dorso-ventrally, but considerably expanded laterally 

 On the whole, it is funnel-shaped, and is bent on itself owing to the cephalic am 

 cervical flexures. The cavity soon becomes irregular, due to the development oi 

 the visceral pouches and certain other evaginations. Four visceral pouches art 

 present by the fifteenth day (fig. 200), and between them the branchial arches 

 show as rounded ridges in the interior of the pharynx. All the pouches have ventral 

 prolongations on to the ventral wall of the pharynx, and all except the fourth have 

 also dorsal pockets. The ventral prolongation of the first pouch reaches farthest 

 towards the mid- ventral line, joining the groove round the tuberculum impar 

 (see below). The remaining pouches do not reach the floor of the pharynx. Each 

 visceral pouch corresponds to an ectodermic visceral cleft. The external cleft and 

 internal pouch are at first separated by mesenchyme ; but later this disappears, 

 and the ectoderm and entoderm come together on the lateral aspect of the pharynx 

 to form a thin septum between them. It is this septum which is broken through 

 in gill- breathing animals to form the gill-cleft, but it is probable that the pharynx 

 does not communicate with the exterior at any stage in mammals. 



The branchial arches in the fish develop gill-filaments containing capillary loops connecting 

 the afferent and efferent branches of the aortic arches. No such filaments are developed in the 

 higher vertebrates, and it might be expected that, with the loss of gill- respiration the gill-pouches 

 would also have disappeared. They are retained to provide the cellular rudiments of certain 

 important organs which arise from the epithelium of the gill-pouches in all vertebrates (Maurer). 



Owing to the great expansion of the mandibular and hyoid arches, the funnel- 

 shape of the pharynx becomes more pronounced as it narrows behind into the 

 oesophagus, and the hinder pouches take a nearly horizontal direction owing to the 

 manner in which the hinder arches are telescoped within the hyoid arch (fig. 201). 

 At first neither the branchial arches nor the visceral pouches reach the mid-ventral 

 line, so that the floor of the cavity is a flat plate overlying the developing heart. 

 On this flat surface a depression appears opposite the third and fourth arches, which 

 is the rudiment of the respiratory passages. This is bounded in front by a transverse 

 ridge joining the ventral ends of the third arches, and on each side by lateral ridges, 



