2l8 



CHORDATE ANATOMY 



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

 nasal passage above. 



Development. At a relatively late state of ontogenesis, at the anterior 

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

 invaginates to form the stomodeum. At the bottom of the stomodeum, 

 ectoderm and endoderm are in contact as a two-layered membrane, which 

 ruptures and disappears leaving no trace in the adult. The covering of the 

 lips and gums is derived from the ectodermal stomodeum, while that of the 

 rest of the mouth is endodermal. 



Evolution. There is no doubt that the mouths of all vertebrates are 

 homologous, the sucking mouth of cyclostomes being no exception. Cyclo- 



epiphysis 



lateral telencephalic 

 vesicle 



hyoid arch 

 visceral arch III 



Fig. 205. — Drawing to show the external appearance of the structures in the oral 

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

 Chick.") 



stome and gnathostome mouths have the same fundamental structure, 

 development, and relations to other parts, and must therefore be considered 

 homologous. 



Beard and Kupffer, however, are persuaded that vertebrates have had 

 two mouths — an old paleostoma and new neostoma. The paleostoma, in 

 their opinion, may be represented by the hypophysis, which in some 

 cyclostomes, e.g. Bdellostoma, opens directly into the pharynx. (See 

 Fig. 206, A) According to Kupffer, the hypophysis of vertebrates repre- 

 sents a paleostoma which functioned as a mouth in prechordates, following 

 their abandonment of the original blastoporic mouth. In support of this 

 assumption, he points out that the definitive mouth of vertebrates arises 

 late in ontogenesis in such relation to the series of gill-slits that it might 

 have been formed from a pair of coalesced gill-sUts; that the presence of a 



