V.] THE NASAL PITS. 117 



lymph. In the region of the cochlea this excavation of the 

 mesoblast takes place along two definite tracts, and thus the 

 two scalse are established ; but in the region of the vestibule 

 and its appendages it is more irregular, resulting in one 

 common cavity interrupted at various points by bridles of 

 connective tissue passing from the osseous to the membranous 

 labyrinth. 



Further details, especially concerning the histological 

 changes, we propose to reserve for a later part of this 

 work. Meanwhile we may remark that all the minute 

 auditory apparatus, the hair-cells, and their various adjuncts, 

 appear to be of a distinctly epithelial nature and of epiblastic 

 origin. In the bird, as is well known, there are no rods of 

 Corti; but even these structures seem in the mammal to be 

 similarly epiblastic. 



It will be seen then that the ear, while resembling the 

 eye in so far as the peculiar structures in which the sensory 

 nerve in each case terminates are formed of involuted epi- 

 blast, differs from it inasmuch as it arises by an independent 

 involution of the superficial epiblast, whereas the eye is a 

 constricted portion of the general involution which gives rise 

 to the central nervous system. Still greater is the distinction 

 between the optic and auditory nerves. The optic nerve is, 

 as we have seen, the consolidated stalk of the optic vesicle, 

 and therefore is of purely epiblastic origin. The auditory 

 nerve, on the contrary, as we shall see, arises in and is formed 

 out of the mesoblast, lying by the side of the otic vesicle. It 

 with its ganglion may readily be recognised in sections as 

 quite independent, both of the otic vesicle and the hind- 

 brain, though subsequently coming into connection with 

 each of them. The growth of the auditory nerve into the 

 hind-brain is shewn in Fig. 34, N; the union of the nerve- 

 fibres with the epithelial structures of the membranous 

 labyrinth takes place at a later date. 



13. At the under surface of each of the vesicles of the 

 cerebral hemispheres there appears towards the end of the 

 third day a small spmewhat elongated vesicle, the olfactory 

 vesicle, which is the rudiment of the olfactory nerve or bulb. 

 Over each olfactory vesicle the external epiblast which covers 

 them grows inwards to form a shallow depression with a 

 thickened border. These depressions are the nasal pits (Fig. 



