EXPLAXATIOX OF PLATE XV. 



Complete list of references. 



cer. cerebral hemispheres. ch. notoehord. 



I V. lateral ventricle. p cl. posterior cliuoid. 



jjn. pineal gland. mn, mandible. 



X. rudiment of septum which will grow backwards and divide the 

 unpaired cerebral rudiment into the two hemispheres. 

 cin. internal carotid. op.n. optic nerve. 



ojj. eye. in. infundibulum. /^^. pituitary body. 



ol. olfactory pit, ol. I. olfactory lobe. 



ojy. th. optic thalamus. p c. posterior commissure. 



m h. mid-brain, or optic lobes. ch. cerebellum. 



md. medulla oblongata. iv. v. fourth ventricle. 



/^. fasciculi teretes. r t. restiform tracts. 



t c. tela vasculosa of the roof of the fourth ventricle. 

 au. V. auditory vesicle. vii. seventh nerve. 



Fig. la, 1 b, 1 c. Longitudinal sections of the brain of a Scyllium 

 embryo belonging to stage L. Zeiss a, ocul. 1. 



la is taken slightly to one side of the middle line, and shews the 

 general features of the brain, and more especially the infundibulum (in.) 

 and pituitary body {j^t). 



1 & is through the median line of the pineal gland, 



1 c is through the median line of the base of the brain, and shews the 

 notoehord {ch.) and pituitary body (pt); the latter still communicating with 

 the mouth. It also shews the wide opening into the infundibulum in the 

 middle line of the base of the brain. 



Fig. 2. Section through the unpaired cerebral rudiment during stage 

 0, to shew the origin of the olfactory lobe and the olfactory nerve. The 

 latter is seen to divide into numerous branches, one of which passes into 

 each Sclmeiderian fold. At its origin are numerous ganglion cells repre- 

 sented by dots. Zeiss a, ocul. 2, 



Fig. 3. Horizontal section through the three lobes of the brain during 

 stage 0. Zeiss a, ocul. 2. 



The figure shews (1) the very slight indications which have appeared 

 by this stage of an ingrowth to divide the cerebral rudiment into two 

 lobes, x. (2) The optic thalami united by a posterior commissure, and on 

 one side joining the base of the mid-brain, and behind them the i)ineal 

 gland, (3) The thin posterior wall of the cerebral rudiment with folds 

 projecting into the cerebral cavity. 



Fig. 4: a, 4b, 4c. Views from the side, from above, and from below, of 

 a brain of Scyllium canicula during stage P. In the view from the side 

 the eye (op.) has not been removed. 



The bilobed appearance both of the mid-brain and cerebellum should 

 be noticed. 



Fig. 5, Longitudinal section of a brain of Scyllium canicula during 

 stage P. Zeiss a, ocul. 2. 



