440 



EXTERNAL AND INTERNAL NARES. 



Fll 



'-.a 



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 dis- 

 tance from the mouth, and placed, by 

 the growth of the snout, on the upper 

 surface of the head (figs. 54 and (J8). 

 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 poste- 

 rior 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 Elasmobianchii) or is quite 

 separate from the mouth (Teleostei and 

 Ganoidei). In the Dipnoi, Amphibia and 

 all the higher types the oral region be- 

 comes extended so as to enclose the pos- 

 terior nares, and then each nasal pit ac- 

 quires two openings ; viz. one outside the 

 mouth, the external nares, and one within 

 the mouth, the internal 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 onto- 

 genetic process by which 

 the posterior nasal open- 

 ing is formed has been 

 studied in the Amniota 

 and Amphibia. Amongst 

 the Amniota we may 

 take the Chick as repre- 



ol 



FIG. 308. SIDE VIEW OF THE 

 HEAD OF AN EMBRYO CHICK OF THE 

 THIRD DAY AS AN OPAQUE OBJECT. 

 (Chromic acid preparation.) 



C.H. cerebral hemispheres ; 

 F. H. vesicle of third ventricle; 

 MI!, mid-brain ; Cfe. cerebellum ; 

 HB. medulla oblongata; N. na- 

 sal pit; ot. auditory vesicle in the 

 stage of a pit with the opening not 

 yet closed up ; op. optic vesicle, 

 with I. lens and ch.f. choroidal 

 fissure. 



1 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. 



FIG. 309. SECTION THROUGH THE BRAIN AND OL- 

 FACTORY ORGAN OF AN EMBRYO OF ScYLLIUM. (Modi- 

 fied from figures by Marshall and myself.) 



c.li. cerebral hemispheres ; o/.r. olfactory vesicle ; 

 olf. olfactory pit ; Feb. Schneiderian folds; /. olfac- 

 tory nerve. ' The reference line has been accidentally 

 taken through the neive to the brain. 



