20 DR. A. MILNES MARSHALL. 



seventy-five hours,i at which period the olfactory pits are deep 

 and conspicious depression, and the olfactory nerves, which are in 

 continuity with the walls of the pits, arise from the front of the 

 fore brain just at its juncture with the commencing cerebral 

 hemispheres. By the subsequent growth of the brain the olfac- 

 tory nerves are separated somewhat from one another, while the 

 growth of the hemispheres upwards and forwards soon causes the 

 olfactory nerves to spring from their ventral surface. 



Figs. 17 — 20 illustrate the condition of the olfactory nerves 

 and surrounding parts in a ninety-three-hours' chick. The plane 

 of the sections is readily understood by referring to the figure of 

 a four-days' chick given by Foster and Balfour." If a line be 

 drawn through the centre of the optic lens, parallel to the sides 

 of the page, it will indicate almost exactly the line along which 

 the section in fig. 17 is taken. Such a section passes through 

 the olfactory pits [olf] ; the fore brain {f h) ; the centre of the 

 optic cup {o c) ; and lens [l] ; the infundibulum {inf) ; the 

 forward diverticulum from the mouth involution to form the 

 pituitary body [pit) ; the ophthalmic branch of the fifth (V) ; 

 the notochord (») ; and the anterior part of the hind brain 



Fig. 18 is taken a little to the right of the line indicated, and 

 passes through the lens near its upper margin ; it passes above 

 the olfactory pits, but through the olfactory nerves (I). The 

 fore brain bulges slightly laterally, forming the commencement 

 of the cerebral hemispheres, with which the olfactory nerves are in 

 close relation. The section also passes through the constriction 

 [mh b), separating the hind from the mid brain. 



Fig. 19 represents part of one of the same series of sections, 

 in a plane slightly dorsad of fig. 18 ; it passes through the points 

 of origin of the olfactory nerves (I) from the fore brain, or, more 

 strictly, from the ventral surface of the cerebral hemispheres [c A). 



From this period the olfactory nerves gradually acquire the 

 length and relations characteristic of the adult. I have followed 

 the changes as far as the eighth day, at which period the nerves 

 are still short, and have also carefully examined the nerves in 

 the adult. 



I have previously shown that " there is no trace whatever of 

 an olfactory vesicle in the early stages : "^ this statement can now 

 be extended : t/icre is no trace of an olfactory vesicle at any 

 period in the life of a chick. 



Thus the direct embryological evidence, though, as I have 

 already pointed out, not absolutely conclusive, yet s])eaks very 



^ Loc. cit., plate xxi, figs. 14, 15. 

 = Op. cit., fig. 40, p. 142. 

 ^ Loc. cit., p. 512. 



