308 DR. A. MILNES MARSHALL. 



spherical ganglionic cells at intervals ; distally, at its connec- 

 tion with the olfactory pit, it presents a second ganglionic 

 swelling, fig. 7. 



A point of very considerable interest, shown in the clearest 

 possible manner by these figures, is that up to this date there 

 is no indication of an olfactory lobe ; indeed, instead of a 

 hollow process of the hemisphere at the point of origin of 

 the olfactory nerve, there is at this point, as is shown by 

 both figures, but especially by fig. 8, a slight external de- 

 pression, with a very obvious internal projection of the wall 

 of the hemisphere. 



Fig. 9 represents a similar section, in a longitudinal and 

 vertical plane, through the nasal region of a chick at the 

 end of the seventh day ; passing through the cerebral hemi- 

 sphere (c. h.), the eye (o. c), the anterior extremity of the oph- 

 thalmic branch of the fifth nerve (v. a.), the olfactory pit (olf.), 

 and the olfactory nerve (i). The nerve itself presents the 

 same histological characters as in fig. 7, /. e.[a. proximal gan- 

 glionic enlargement at its root of origin, a trunk consisting 

 mainly of nerve fibres, but with a few ganglionic cells at inter- 

 vals along its whole length, and a distal ganglionic expansion 

 at its point of fusion with the olfactory epithelium. There is, 

 however, one important difference between this figure and 

 the two preceding ones ; opposite the point of origin of the 

 olfactory nerve there is a small conical depression (ol. v.) of the 

 inner wall of the cerebral hemisphere. From a comparison 

 with fig. 20 there can be little doubt that this is the earliest 

 appearance of an olfactory lobe. As in the dogfish, this 

 lobe is formed at first entirely at the expense of the inner 

 wall of the hemisphere, there being as yet no perceptible 

 projection on the exterior of the brain. 



The olfactory lobes, after their first appearance, grow ra- 

 pidly. By the twelfth day they form a pair of small conical 

 processes, about 1 millimetre in length, springing from the 

 extreme anterior ends of the cerebral hemispheres : the two 

 lobes lie side by side, their apposed surfaces being slightly 

 flattened. Each lobe contains a prolojigation of the ventri- 

 cular cavity of the corresponding hemisphere. 



Fig 36 represents a longitudinal and vertical section 

 through the olfactory lobe and the anterior part of the cere- 

 bral hemisphere of a twelfth day chick embryo : it shows 

 how the ventricle of the hemisphere (c.h.) is prolonged to 

 the extremity of the olfactory lobe (ol.v.) ; and also the mode 

 in which the olfactory nerve arises from the end of the 

 olfactory lobe as a series of bundles of nerve fibres. 



Fig. 37 is a transverse section through the olfactory nerve 



