EXPLANATION OF PLATES 15, 1 6, I/. xxiii 



Fig. 4. Section through the dorsal region of a Torpedo embryo slightly older 

 than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The section shews the 

 formation of a pair of dorsal nerve-rudiments (/ ;) and a ventral nerve-rudiment (a r). 

 The latter is shewn in its youngest condition, and is not distinctly cellular. 



Fig. 5. Section through the dorsal region of a Torpedo embryo slightly younger 

 than stage K. (Zeiss CC, ocul. 2.) The connective-tissue cells are omitted. The 

 rudiment of the ganglion (spg) on the posterior loot has appeared, and the junction of 

 posterior root with the cord is difficult to detect. The anterior root forms an elonga- 

 ted cellular structure. 



Fig. 6. Section through the dorsal region of a Pristiurus embryo of stage K. 

 (Zeiss CC, ocul. 2.) The section especially illustrates the attachment of the posterior 

 root to the spinal cord. 



Fig. 7. Section through the same embryo as fig. 6. (Zeiss CC, ocul. i.) The 

 section contains an anterior root, which takes its origin at a point opposite the interval 

 between two posterior roots. 



Fig. 8. A series of posterior roots with their central ends united by a dorsal 

 commissure, from a longitudinal and vertical section of a Scyllium embryo belonging 

 to a stage intermediate between L and M. The embryo was hardened in a mixture 

 of osmic and chromic acids. 



Fig. 9. The central end of a posterior nerve-root from the same embryo, with the 

 commissure springing out from it on either side. 



EXPLANATION OF PLATES 15, 16, 17. (X. p. 397.) 



PLATE 15. (THE HEAD DURING STAGES G K.) 

 COMPLETE LIST OF REFERENCE LETTERS. 



laa, laa, etc. ist, 2d, etc. aortic arch. acv. Anterior cardinal vein. a/. Ali- 

 mentary canal, ao. Aorta. an. Thickening of epiblast to form the auditory pit. 

 aun. Auditory nerve, aup. Auditory pit. auv. Auditory vesicle, b. Wall of 

 brain. bb. Base of brain, cb. Cerebellum. c<?r. Cerebrum. Ch. Choroid slit. 

 ch. Notochord. com. Commissure connecting roots of vagus nerve, i, 2, 3 etc. 

 eg. External gills, ep. External epiblast. fb. Fore-brain, gl. Glossopharyngeal 

 nerve, h b. Hind-brain, ht. Heart, hy. Hyaloid membrane. In. Infundibulum. 

 /. Lens. M. Mouth involution, m. Mesoblast at the base of the brain, m b. Mid- 

 brain, inn. v. Mandibular branch of fifth, ol. Olfactory pit. op. Eye. op n. Optic 

 nerve, opv. Optic vesicle, opth v. Ophthalmic branch of fifth, p. Posterior root 

 of spinal nerve, pn. Pineal gland. 1,2 etc. pp. First, second, etc. section of body- 

 cavity in the head. pt. Pituitary body. so. Somatopleure. sp. Splanchnopleure. 

 spc. Spinal cord. Th. Thyroid body. v. Blood-vessel, iv. v. Fourth ventricle, 

 v. Fifth nerve. V ' c. Visceral cleft. Vg. Vagus, vii. Seventh or facial nerve. 



Fig. i . Head of a Pristiurus embryo of stage K viewed as a transparent object. 



The points which deserve special attention are: (i) The sections of the body- 

 cavity in the head (pp) : the first or premandibular section being situated close to the 

 eye, the second in the mandibular arch. Above this one the fifth nerve bifurcates. 

 The third at the summit of the hyoid arch. 



