XXVI EXPLANATION OF PLATE 1 6. 



The pituitary body still opens into the mouth, though the septum between the 

 mouth and the throat is broken through. 



16 b is a more superficial section shewing the head-cavities// i, 2, 3, and the 

 lower vagus commissure. 



PLATE 16. (X. p. 397.) 



COMPLETE LIST OF REFERENCE LETTERS. 



au v. Auditory vesicle, cb. Cerebellum, cer. Cerebral hemispheres, ch. Noto- 

 chorcl. cin. Internal carotid. ft. Fasciculi teretes. in. Infundibulum. lv. 

 Lateral ventricle, m b. Mid-brain, or optic lobes, md. Medulla oblongata. mn. 

 Mandible. ol. Olfactory pit. oil. Olfactory lobe. op. Eye. op n. Optic nerve. 

 opth. Optic thalamus. pc. Posterior commissure, pel. Posterior clinoid. pn. 

 Pineal gland, pt. Pituitary body, r t. Restiform tracts, i v. Tela vasculosa of the 

 roof of the fourth ventricle, iv. v. Fourth ventricle, vii. Seventh nerve, x. Rudi- 

 ment of septum which will grow backwards and divide the unpaired cerebral rudiment 

 into the two hemispheres. 



Figs, i a, \ b, ic. Longitudinal sections of the brain of a Scyllium embryo 

 belonging to stage L. Zeiss a, ocul. i. 



r a 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 (pt). 



i b is through the median line of the pineal gland. 



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

 (ck) and pituitary body (pt) ; the latter still communicating with the mouth. It also 

 shews the wide opening of the infundibulum in the middle line into the base of the 

 brain. 



Fig. 2. Section through the unpaired cerebral rudiment during stage O, 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 Schneiderian fold. At its 

 origin are numerous ganglion cells represented by dots. Zeiss a, ocul. 2. 



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

 Zeiss a, ocul. 2. 



The figure shews (i) 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 pineal gland : (3) the thin posterior wall of the 

 cerebral rudiment with folds projecting into the cerebral cavity. 



Figs. 40, 4 b, 4<r. 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. 



