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 root has appeared, and the junction of posterior root with the cord is difficult to detect. The anterior root forms an elongated 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. 1.) 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. 1_aa_, 2_aa_, etc. 1st, 2d, etc. aortic arch. _acv._ Anterior cardinal vein. _al._ Alimentary canal. _ao._ Aorta. _au._ 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. _cer._ Cerebrum. _Ch._ Choroid slit. _ch._ Notochord. _com._ Commissure connecting roots of vagus nerve. 1, 2, 3 etc. _eg._ External gills. _ep._ External epiblast. _fb._ Fore-brain. _gl._ Glossopharyngeal nerve. _hb._ Hind-brain. _ht._ Heart. _hy._ Hyaloid membrane. _In._ Infundibulum. _l._ Lens. _M._ Mouth involution. _m._ Mesoblast at the base of the brain. _mb._ Mid-brain. _mn._ v. Mandibular branch of fifth. _ol._ Olfactory pit. _op._ Eye. _opn._ 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. _Vc._ Visceral cleft. _Vg._ Vagus. vii. Seventh or facial nerve. Fig. 1. Head of a Pristiurus embryo of stage K viewed as a transparent object. The points which deserve special attention are: (1) 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. The cranial nerves and the general appearance of the brain are well shewn in the figure.