Figs. 4_a_, 4_b_, 4_c_. 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. There should be noticed (1) the increase in the flexure of the brain accompanying a rectification of the cranial axis; (2) the elongated pineal gland, and (3) the structure of the optic thalamus. Figs. 6_a_, 6_b_, 6_c_. Views from the side, from above, and from below, of a brain of Scyllium stellare during a slightly later stage than Q. Figs. 7_a_ and 7_b_. Two longitudinal sections through the brain of a Scyllium embryo during stage Q. Zeiss a, ocul. 2. 7_a_ cuts the hind part of the brain nearly through the middle line; while 7_b_ cuts the cerebral hemispheres and pineal gland through the middle. In 7_a_ the infundibulum (1), cerebellum (2), the passage of the restiform tracts (_rt_) into the cerebellum (3), and the rudiments of the tela vasculosa (4) are shewn. In 7_b_ the septum between the two lobes of the cerebral hemispheres (1), the pineal gland (2), and the relations of the optic thalami (3) are shewn. Figs. 8_a_, 8_b_, 8_c_, 8_d_. Four transverse sections of the brain of an embryo slightly older than Q. Zeiss a, ocul. 1. 8_a_ passes through the cerebral hemispheres at their junction with the olfactory lobes. On the right side is seen the olfactory nerve coming off from the olfactory lobe. At the dorsal side of the hemispheres is seen the pineal gland (_pn_). 8_b_ passes through the mid-brain now slightly bilobed, and the opening into the infundibulum (_in_). At the base of the section are seen the optic nerves and their chiasma. 8_c_ passes through the opening from the ventricle of the mid-brain into that of the cerebellum. Below the optic lobes is seen the infundibulum with the rudiments of the sacci vasculosi. 8_d_ passes through the front end of the medulla, and shews the roots of the seventh pair of nerves, and the overlapping of the medulla by the cerebellum. PLATE 17. (X. p. 397.) COMPLETE LIST OF REFERENCE LETTERS.