328 OUTLINES OF CHORDATE DEVELOPMENT 



before the formation of any other landmark. The maxillary 

 and mandibular arches, whose formation is described below, 

 extend forward, around the sides of the stomodaeum forming 

 the rudiments of the jaws; the buccal cavity is considerably 

 enlarged by their formation and their enlargement as the beak. 

 An incomplete palate is formed later, above which the pharyn- 

 geal cavity extends. On the upper surface of the beak is 

 formed a superficial horny egg-tooth, which is used in perforat- 

 ing the shell and shell membrane at the close of incubation; 

 and on the margins of the jaws slight, transitory ridges appear, 

 representing the vestiges of the enamel organ all that there 

 is left of the true teeth of other vertebrates. Although the 

 tongue extends forward into the buccal cavity, it is really a 

 pharyngeal derivative . 



The pharynx is the most complicated region of the em- 

 bryonic gut. On account of the obliquity of the oral plate, 

 the antero-dorsal portion of the pharynx may be described as 

 pre-oral; this region is known as SeesseWs pouch, but when the 

 oral plate disappears this is indistinguishable (Figs. 123, A; 

 130). We have already described, in connection with the ear, 

 the separation of this antero-dorsal portion of the pharynx as 

 the rudiment of a part of the tympanic cavity and the Eusta- 

 chian tubes, which open into the pharynx through a median 

 fissure in the palate, the tubal fissure. 



Beginning the second day there grow out from the walls of 

 the wide pharynx toward the surface of the head, a series of 

 paired, vertically elongated pouches, the visceral pouches (Fig. 

 130). Of these there are four pairs, diminishing in size and 

 importance ppsteriorly. The first and largest is the hyoman- 

 dibular pouch, the other three are the branchial or gill-pouches, 

 the last of which is very feebly developed. These visceral 

 pouches, containing extensions of the pharyngeal cavity, push 

 out to the surface ectoderm with which they fuse intimately, 

 dividing the body wall of the region into a series of vertical 

 sections between them; these are the visceral arches. The 

 visceral arches are composed chiefly of mesenchyme, in which 

 develop later the aortic arches and the cartilaginous visceral 



