THE DIGESTIVE SYSTEM 



205 



by the hard and soft palates, which separate the mouth cavity from the 

 narial passage above. 



Development of the Mouth 



At a relatively late stage of ontogenesis, at the anterior end of the 

 fore-gut where the mouth is to break through, the ectoderm invaginates to 

 form the stomodeum, the depth of which, in amniotes, is considerably 

 increased by the expansion of the fore-brain in front and the heart behind. 

 At the bottom of the stomodeum, ectoderm and endoderm are in contact 

 as a two-layered membrane, which ruptures and disappears leaving no 



dicnccphalon 



epiphysis 



lateral telencephalic 

 vesicle 



hyomandibolar cleft 



hyoid arch 

 visceral arch in 



Fig. 247. — Drawing to show the external appearance of the structures in the oral 

 region of a four-day chick. Ventral aspect. (From Patten's " Embrvology of the 

 Chick.") 



trace in the adult. Thus, in forming the mouth, the ectoderm takes the 

 initiative, whereas the gill-slits are primarily outpocketings of the 

 endoderm. 



The covering of the lips and gums is derived from the ectodermal 

 stomodeum, while that of the rest of the mouth is endodermal. The 

 salivary glands arise from the epithelial lining of the mouth and are 

 generally supposed to be ectodermal in origin. The enamel of the teeth 

 is a product of the stomodeal wall, and therefore, although within the 

 digestive cavity, ectodermal. 



Evolution of the Mouth 



There is no doubt that the mouths of all vertebrates are homologous, 

 the sucking mouth of cyclostomes being no exception. Cyclostome and 

 gnathostome mouths have the same fundamental structure, development, 

 and relations to other parts, and must therefore be considered homologous. 



