Part 13

1c 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.

Figs. 4a, 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.

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. 6a, 6b, 6c. Views from the side, from above, and from below, of a brain of Scyllium stellare during a slightly later stage than Q.

Figs. 7a and 7b. Two longitudinal sections through the brain of a Scyllium embryo during stage Q. Zeiss a, ocul. 2.

7a cuts the hind part of the brain nearly through the middle line; while 7b cuts the cerebral hemispheres and pineal gland through the middle.

In 7a 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 7b 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. 8a, 8b, 8c, 8d. Four transverse sections of the brain of an embryo slightly older than Q. Zeiss a, ocul. 1.

8a 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).

8b 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.

8c 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.

8d 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.

vii.a. Anterior branch of seventh nerve. ar. Anterior root of spinal nerve. auv. Auditory vesicle. cer. Cerebrum. ch. Notochord. ch. Epithelial layer of choroid membrane. gl. Glossopharyngeal nerve. vii.hy. Hyoid branch of seventh nerve. hym. Hyaloid membrane. ll. Lateral line. v.mn. Ramus mandibularis of fifth nerve. vii.mn. Mandibular (spiracular) branch of seventh nerve. v.mx. Ramus maxillæ superioris of fifth nerve. nl. Nervus lateralis. ol. Olfactory pit. op. Eye. v.opth. Ramus ophthalmicus of fifth nerve. pch. Parachordal cartilage. pfal. Processus falciformis. pp. Head cavity. pr. Posterior root of spinal nerve. rt. Retina. sp. Spiracle. v. Fifth nerve. vii. Seventh nerve. vc. Visceral cleft. vg. Vagus nerve. vg.br. Branchial branch of vagus. vgcom. Commissure uniting the roots of the vagus, and continuous with commissure uniting the posterior roots of the spinal nerves. vgr. Roots of vagus nerves in the brain. vgin. Intestinal branch of vagus. vh. Vitreous humour.

Fig. 1. Diagram of cranial nerves at stage L.

A description of the part of this referring to the vagus and glossopharyngeal nerves is given at p. 426. It should be noticed that there are only five strands indicated as springing from the spinal cord to form the vagus and glossopharyngeal nerves. It is however probable that there are even from the first a greater number of strands than this.

Fig. 2. Section through the hinder part of the medulla oblongata, stage between K and L. Zeiss A, ocul. 2.

It shews (1) the vagus commissure with branches on one side from the medulla: (2) the intestinal branch of the vagus giving off a nerve to the lateral line.

Fig. 3. Longitudinal and vertical section through the head of a Scyllium embryo of stage L. Zeiss a, ocul. 2.

It shews the course of the anterior branch of the seventh nerve (vii.); especially with relation to the ophthalmic branch of the fifth nerve (v.oth).

Figs. 4a and 4b. Two horizontal and longitudinal sections through the head of a Scyllium embryo belonging to stage O. Zeiss a, ocul. 2.

4a is the most dorsal of the two sections, and shews the course of the anterior branch of the seventh nerve above the eye.