VI.] THE OLFACTORY ORGAN. 161 



whereas the eye is a constricted portion of the general 

 involution which gives rise to the central nervous 

 system. 



The origin of the auditory nerve has already been 

 described. It is shewn in close contact with the walls 

 of the auditory pit in Fig. 53. 



Organ of Smell. The organ of smell makes its ap- 

 pearance during the third day, as two depressions or 

 pits, on the under surface of the head, a little in front 

 of the eye (Fig. 56, N). 



Head of an Embryo Chick op the Third Day viewed 



sideways as an opaque object. 



(Chromic acid preparation.) 



C.H. Cerebral hemispheres. F.B. Vesicle of third ventricle. 

 M.B. Mid-brain. Cb. Cerebellum. H.B. Medulla ob- 

 longata, 



N. Nasal pit. ot. otic vesicle in the stage of a pit with the open- 

 ing not yet closed up. op. Optic vesicle, with I. lens and 

 oh.f. choroidal fissure. The superficial epiblast moulds 

 itself to the form of the optic vesicle and the lens ; hence 

 the choroidal fissure, though formed entirely underneath the 

 superficial epiblast, is distinctly visible from the outside. 



1 F. The first visceral fold; above it is seen a slight indication of 

 the superior maxillary process. 



2, 3, 4 F. Second, third and fourth visceral folds, with the vis- 

 ceral clefts between them. 

 F. &B. 11 



