1 88 Vertebrate Embryology 



triangular part of the face, between the 

 two nasal grooves) and the maxillary arch 

 (forming the upper jaw), becomes deeper, 

 and, during the fifth day, is converted into 

 a closed tube, by the fusion of its edges, 

 the above-mentioned fronto-nasal process and 

 maxillary arch. The groove is thus convert- 

 ed into a tube leading from the nose into 

 the front part of the mouth cavity. By the 

 forward growth of the beak, and the for- 

 mation of a horizontal septum, the palatine 

 bone, the opening of this tube, the posterior 

 nares, comes to lie in the back rather than 

 in the front part of the oral cavity (Fig. 64.) 



In the development of the sense organs, 

 the eye, ear, and nose, there are many points 

 in common, but it should be especially noted 

 that in all three of these organs the essen- 

 tially sensory parts, in each case, are derived 

 either directly or indirectly from the ectoblast. 



The Visceral Clefts and Arches. Owing to 

 the unequal rates of growth of the different 

 parts of the chick, the relative positions of 

 the various organs are constantly changing. 

 During the second day, it will be remembered, 

 the heart was formed in the mesoblast, under 

 the anterior end of the digestive tract or mes- 



