15_b_ mainly illustrates the dorsal commissure of the vagus nerve (_com_) continuous with the dorsal commissures of the posterior root of the spinal nerves. Fig. 16. Two longitudinal and vertical sections of the head of a Pristiurus embryo belonging to the end of stage K. Zeiss a, ocul. 1. 16_a_ passes through the median line of the brain and shews the infundibulum, notochord and pituitary body, etc. 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 _pp_ 1, 2, 3, and the lower vagus commissure. PLATE 16. (X. p. 397.) COMPLETE LIST OF REFERENCE LETTERS. _auv._ Auditory vesicle. _cb._ Cerebellum. _cer._ Cerebral hemispheres. _ch._ Notochord. _cin._ Internal carotid. _ft._ Fasciculi teretes. _in._ Infundibulum. _lv._ Lateral ventricle. _mb._ Mid-brain, or optic lobes. _md._ Medulla oblongata. _mn._ Mandible. _ol._ Olfactory pit. _oll._ Olfactory lobe. _op._ Eye. _opn._ Optic nerve. _opth._ Optic thalamus. _pc._ Posterior commissure. _pcl._ Posterior clinoid. _pn._ Pineal gland. _pt._ Pituitary body. _rt._ Restiform tracts. _tv._ Tela vasculosa of the roof of the fourth ventricle. iv._v._ Fourth ventricle. vii. Seventh nerve. _x._ Rudiment of septum which will grow backwards and divide the unpaired cerebral rudiment into the two hemispheres. Figs. 1_a_, 1_b_, 1_c_. Longitudinal sections of the brain of a Scyllium embryo belonging to stage L. Zeiss a, ocul. 1. 1_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_). 1_b_ 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 notochord (_ch_) 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 (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 pineal gland: (3) the thin posterior wall of the cerebral rudiment with folds projecting into the cerebral cavity.