OLFACTORY ORGANS. 



535 



The opening of the nasal pit becomes surrounded by a ridge 

 except on its oral side. The deficiency of this ridge on the side 

 of the mouth gives rise to a kind of shallow groove leading from 





FIG. 309. SECTION THROUGH THE BRAIN AND OLFACTORY ORGAN OF AN 

 EMBRYO OF ScYLLlUM. (Modified from figures by Marshall and myself.) 



c. h. cerebral hemispheres ; ol.v. olfactory vesicle; olf. olfactory pit ; Sf/i. Schnei- 

 derian folds ; /. olfactory nerve. The reference line has been accidentally taken 

 through the nerve to the brain. 



the nasal pit to the mouth. The ridge enveloping the opening 

 of the nasal pit next becomes prolonged along the sides of this 

 groove, especially on its inner one; and at the same time the 

 superior maxillary process grows forwards so as to bound the lower 



mS 



FIG. 310. DIAGRAMMATIC VERTICAL SECTION THROUGH THE HEAD OF A 



LARVA OF PETROMYZON. 



The larva had been hatched three days, and was 4'8 mm. in length. The optic 

 and auditory vesicles are supposed to be seen through the tissues. 



c.h. cerebral hemisphere ; th. optic thalamus; in. infundibulum; pn. pineal gland ; 

 mb. mid-brain; cb. cerebellum; md. medulla oblongata; an.v. auditory vesicle; op. 

 optic vesicle; ol. olfactory pit; in. mouth; br.c. branchial pouches; th. thyroid 

 involution; v.ao. ventral aorta; ht. ventricle of heart ; ch. notochord. 



