444 DEVELOPMENT OF ELASMOBRANCH FISHES. 



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 ib cuts 

 the cerebral hemispheres and pineal gland through the middle. 



In 70 the infundibulum (i), 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 (i), the pineal gland (2), 

 and the relations of the optic thalami (3) are shewn. 



Figs. 8 a, 8 b, 8c,8d. Four transverse sections of the brain of an embryo slightly 

 older than Q. Zeiss a, ocul. i . 



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. 

 COMPLETE LIST OF REFERENCE LETTERS. 



vii. a. Anterior branch of seventh nerve, a r. Anterior root of spinal nerve. 

 au v. 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. //. Lateral line. v. mn. Ramus mandibularis 

 of fifth nerve, vii. mn. Mandibular (spiracular) branch of seventh nerve, v. mx. 

 Ramus maxillae superioris of fifth nerve, n I. Nervus lateralis. ol. Olfactory pit. 

 op. Eye. v. op th. Ramus ophthalmicus of fifth nerve. / ch. Parachordal cartilage. 

 pfal. Processus falciformis. pp. Head cavity, pr. Posterior root of spinal nerve. 

 rt. Retina, sp. Spiracle, v. Fifth nerve, vii. Seventh nerve, v c. Visceral cleft. 

 vg. Vagus nerve, vgbr. 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, v h. Vitreous humour. 



Fig. i. 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. 



