THE NASAL CAVITIES AND OLFACTOBY STRUCTURES. 



21 



(Fig. 16), the plates being depressed to form the olfactory pits. 

 With the growth of the nasal processes the olfactory plates and 

 pits are thrust into the roof of the stomodaeum, where they form 

 the epithelial lining of the upper third or olfactory area of the nasal 

 cavities. A small island is detached from each plate to form the 

 basis of Jacobson's organ (Fig. 16). The sense epithelia send out 

 nerve processes which form arborescences round the neural cells of 

 the outgrowing olfactory lobe (Fig. 17). The olfactory nerves are 

 thus formed. At first the olfactory plate is directly in contact with 

 the neural tube, and it is probable that neuroblasts may migrate 

 then to the olfactory plate and form the olfactory sense epithelium. 



The condition of olfactory pits, a developmental phase in the 

 human embryo, is the permanent form in fishes (Fig. 2 1 i?) ; in 

 amphibians and all higher vertebrates the fundus of the pit 

 breaks down and thus the olfactory pits come into communica- 

 tion through the posterior nares with the stomodaeum (Fig. 8). 



(2) The Olfactory Lobe. — As the olfactory plates are being- 

 thrust downwards, the anterior part of the floor of the fore-brain 

 grows out on each side as a hollow protrusion to form the 



foramen of Monro 



(rudiment 

 of corp. callosum) 



caudate nucleus 



olfact. lobe, ant. part (A) 



olfact. lobe, post, past (B) 



hypopophysis cerebri 



pineal body 



mid brain 



IC 



Ilk- ^r cerebellum 



Fig. 17.— The Mesial aspect of the Brain of a human foetus, 3J months old, showing 

 the Olfactory Lobe. 



olfactory lobes (Fig. 145, p. 178). At the end of the 3rd month 

 the olfactory lobe has assumed the form shown in Fig. 17. Its 

 cavity is at first continuous with that of the cerebral vesicle 

 (lateral ventricle), but this connection is soon lost; it becomes 



