THE EMBRYO. 



1185 



and becomes converted into a channel, which eventually forms the olfactory region 

 of the nose ; this comprises the portion to which the olfactory nerves are distributed. 

 At this time the nasal cavity is continuous with the buccal cavity ; but as the palatal 

 septum is formed, the buccal cavity is divided into two parts, the upper of which 



Ganglion 

 cochleare' '& 



-Vertical semi- 

 circular canal. 



- Utricle. 



.Saccule. 

 .External semi- 

 circular canal. 



IB- -Ductus cochlearis. 



FIG. 739. Transverse section through head of foetal sheep, in the region of the labyrinth. X 30. 

 (After Boettcher.) 



forms the lower pai't of the nasal fossre, while the remainder forms the permanent 

 mouth. On the mesial wall of the nasal fossa a small blind pit of epiblast becomes 

 invaginated and extends backward into the nasal septum. This forms the rudi- 

 ment of Jacobsons organ, which ultimately becomes partly enclosed in a curved 

 cartilaginous plate derived from a cartilage of the nasal septum. 



.^- Embryonic con- 

 e "a nective tissue. 



Canal of 

 \ cochlea. 



i\ 



tx^v-^h 



Epithelium of Corti's organ. 



^Ligamentum 



spirale. 

 ^Scala tympani. 



FIG. 740. Transverse section of the canal of the cochlea of a foetal cat. (After Boettcher and Ayres.) 

 (Fiom Kollmann's Entivickelungsgeschichte.) 



The development of the external nose has already been described. It is 

 perceptible about the end of the second month. The nostrils are at first closed 

 by epithelium, but this disappears about the fifth month. 



The olfactory lobe (rhinencephalon) is formed, as already explained, by an 

 evagination of the anterior cerebral vesicle. 



75 



