Fig. 18. Vertical longitudinal section of a Scyllium embryo belonging to stage K. Zeiss A, ocul. 1. Hardened in a mixture of osmic and chromic acid. It shews (1) the commissures connecting together the posterior roots of the spinal nerves; (2) the junction of the anterior and posterior roots; (3) the relations of the segmental ducts to the segmental involutions and the alternation of calibre in the segmental tube; (4) the germinal epithelium lining the body-cavity. EXPLANATION OF PLATE 13. (X. p. 361.) COMPLETE LIST OF REFERENCE LETTERS. _al._ Alimentary tract. _ao._ Aorta. _c._ Connective tissue. _cav._ Cardinal vein. _ch._ Notochord. _ep._ Epiblast. _ha._ Hæmal arch. _l._ Liver. _ll._ Lateral line. _mc._ Mucous canal of the head. _mel._ Membrana elastica externa. _mp._ Muscle-plate. _mp´._ Muscles of muscle-plate. _na._ Neural arch. _nl._ Nervus lateralis. _rp._ Rib process. _sd._ Segmental duct. _sh._ Sheath of notochord. _spc._ Spinal cord. _spg._ Spinal ganglion. _syg._ Sympathetic ganglion. _um._ Ductus choledochus. _v._ Blood-vessel. _var._ Vertebral arch. _vb._ Vertebral body. _vcau._ Caudal vein. _vin._ Intestinal branch of the vagus. _vop._ Ramus ophthalmicus of the fifth nerve. _x._ Subnotochordal rod. Fig. 1. Section through the anterior part of an embryo of _Scyllium canicula_ during stage L. _c._ Peculiar large cells which are found at the dorsal part of the spinal cord. Sympathetic ganglion shewn at _syg._ Zeiss A, ocul. 1. Fig. 2. Section through the lateral line at the time of its first formation. The cells marked _nl_ were not sufficiently distinct to make it quite certain that they really formed part of the lateral nerve. Zeiss B, ocul. 2. Figs. 3_a_, 3_b_, 3_c_, 3_d_. Four sections of the lateral line from an embryo belonging to stage L. 3_a_ is the most anterior. In 3_a_ the lateral nerve (_nl_) is seen to lie in the mesoblast at some little distance from the lateral line. In 3_b_ and 3_c_ it lies in immediate contact with and partly enclosed by the modified epiblast cells of the lateral line. In 3_d_, the hindermost section, the lateral line is much larger than in the other sections, but no trace is present of the lateral nerve. The sections were taken from the following slides of my series of the embryo (the series commencing at the tail end) 3_d_ (46), 3_c_ (64), 3_b_ (84), 3_a_ (93). The figures all drawn on the same scale, but 3 _a_ is not from the same side of the body as the other sections. Fig. 4. Section through lateral line of an embryo of stage P at the point where it is acquiring an opening to the exterior. The peculiar modified cells of its innermost part deserve to be noticed. Zeiss D, ocul. 2. Fig. 5. Mucous canals of the head with branches of the ramus ophthalmicus growing towards them. Stage O. Zeiss A, ocul. 2.