534 



EXTERNAL AND INTERNAL NARES. 



usually constricted in the middle, and the original single 

 opening of the nasal sack thus becomes nearly divided into two. 

 In Teleostei and Ganoids the division of the nasal opening into 

 two parts becomes complete, but the ventral opening is generally 

 carried off some distance from the mouth, and placed, by the 

 growth of the snout, on the upper surface of the head (figs. 54 

 and 68). In all these instances it is 

 probable that the dorsal opening of 

 the nasal sack is homologous with 

 the external nares, and the ventral 

 opening with the posterior nares of 

 higher types. Thus the posterior 

 nares would in fact seem to be re- 

 presented in all Fishes by a ventral 

 part of the opening of the original 

 nasal pit which either adjoins the 

 border of the mouth (many Elasmo- 

 branchii) or is quite separate from 

 the mouth (Teleostei and Ganoidei). 

 In the Dipnoi, Amphibia and all the 

 higher types the oral region becomes 

 extended so as to enclose the pos- 

 terior nares, and then each nasal pit 

 acquires two openings ; viz. one out- 

 side the mouth, the external nares, 

 and one within the mouth, the in- 

 ternal or posterior nares. In the 

 Dipnoi the two nasal openings are very similar to those in 

 Ganoidei and Teleostei, but both are placed on the under surface 

 of the head, the inner one being within the mouth, and the 

 external one is so close to the outer border of the upper lip that 

 it also has been considered by some anatomists to lie within the 

 mouth. 



In all the higher types the nasal pits have originally only a 

 single opening, and the ontogenetic process by which the 

 posterior nasal opening is formed has been studied in the 

 Amniota and Amphibia. Amongst the Amniota we may take 

 the Chick as representing the process in a very simple form. The 

 general history of the process was first made out by Kolliker. 



FIG. 308. SIDE VIEW OF THE 

 HEAD OF AN EMBRYO CHICK OF 

 THE THIRD DAY AS AN OPAQUE 

 OBJECT. (Chromic acid prepara- 

 tion.) 



C.H. cerebral hemispheres ; 

 F.B. vesicle of third ventricle; 

 M.B. mid-brain; Cb. cerebellum; 

 H.B. medulla oblongata; A^. na- 

 sal pit ; ot. auditory vesicle in the 

 stage of a pit with the opening not 

 yet closed up; op. optic vesicle, 

 with /. lens and ch.f. choroidal 

 fissure. 



i F. The first visceral fold; 

 above it is seen the superior max- 

 illary process. 



2, 3, 4 F. Second, third and 

 fourth visceral folds, with the 

 visceral clefts between them. 



