Bawden, Nose and Jacob sort s Organ. 119 



to vertebrates. They can form new sense-organs only by 

 a further differentiation of the segmentally arranged sense 

 organs which already exist and which have been inherited from 

 the Annelida, their ancestors." That is, he maintains that the 

 nose and ear are comparable in origin, not being modified gill- 

 clefts as formerly supposed, but developed from these simple 

 paired pits which (as in the fish) are found scattered about the 

 head and arranged in a line along the lateral aspect of these lower 

 types. Like the lens and labyrinth of the ear, they are formed from 

 the external epithelium ; unlike them them are never closed 

 up. The nasal epithelium depression which so much resembles 

 in early development the corresponding optic and auditory de- 

 pressions, is developed a little later in the history of the indi- 

 . vidual (that is, in the development, or ontogeny) and like them, 

 from the outer germ-layer. This nasal area lies on either side 

 of the frontal process in the embryo. Early in the process of 

 the differentiation the edges or rims of these depressions rise into 

 folds or ridges. As these thicken there is a corresponding 

 proliferation of neural cells from the brain-tube which come into 

 connection with the cells of these thickened walls. This is 

 what may be called the primitive olfactory ganglionic founda- 

 tion. This evagination of the cerebral vesicle is undoubtedly 

 the fundament of the olfactory lobe, while the fibrillse which 

 terminate in this thickened epithelium are later carried back to 

 form the olfactory bulb and nerve bundles. Thus the nose is all 

 developed, really, by the growth and differentiation of one small 

 piece of sensory epithelium. 



Embryonic development. — The olfactory is connected with 

 and associated with the pituitary in the embryonic development, 

 and as recently shown, they are really situated upon the dorsal 

 surface. The olfactory nerve is supposed by some to be the 

 supra-branchial branch which has every other element suppress- 

 ed. The olfactory nerve is formed from the anterior cerebral as 

 a secondary vesicle on its under surface. Professor Beard says, 

 " In brief the olfactory nerve, or better, the olfactory ganglionic 

 foundation, is derived from two sources, the lateral ganglion and 



