_m._ mesoblast growing up from vertebral rudiment. _sd._ segmental duct. Fig. D c. Section from a still older _Torpedo_-embryo. Camera. (Zeiss CC, ocul. 2.) The connective-tissue cells are omitted. The rudiment of the ganglion (_g_) on the posterior root has appeared. The rudiment of the posterior nerve is much longer than before, and its junction with the spinal cord is difficult to detect. The anterior root is now an elongated cellular structure. _g._ ganglion. Fig. D d. Longitudinal and vertical section through a _Torpedo_-embryo of the same age as D c. The section shews the commissures (_x_) uniting the posterior roots. Fig. E a. Section of a _Pristiurus_-embryo belonging to the second stage. Camera. (Zeiss CC, ocul. 2.) The section shews the constriction of the pedicle which attaches the posterior nerve-rudiments to the spinal cord. _pr._ rudiment of posterior nerve-root. _nc._ neural canal. _mp._ muscle-plate. _vr._ vertebral rudiment. _sd._ segmental duct. _ch._ notochord. _so._ somatopleure. _sp._ splanchnopleure. _ao._ aorta. _al._ alimentary canal. Fig. E b. Section of a _Pristiurus_-embryo slightly older than E a. Camera. (Zeiss CC, ocul. 2.) The section shews the formation of the anterior nerve-root (_ar_). _ar._ rudiment of the anterior nerve-root. Fig. F. Section of a _Pristiurus_-embryo with the rudiments of five visceral clefts. Camera. (Zeiss CC, ocul. 2.) The rudiment of the posterior root is seen surrounded by connective-tissue, from which it cannot easily be distinguished. The artist has not been very successful in rendering this figure. Figs. G 1, G 2, G 3. Three longitudinal and horizontal sections of an embryo somewhat older than F. The embryo from which these sections were taken was hardened 035.png in osmic acid, but the sections have been represented without tinting. G I is most dorsal of the three sections. Camera. (Zeiss CC, ocul. 1.) _nc._ neural canal. _sp.c._ spinal cord. _pr._ rudiment of posterior root. _ar._ rudiment of anterior root. _mp._ muscle-plate. _c._ connective-tissue cells. _ch._ notochord. PLATE 23. Fig. H I. Section through the dorsal region of a _Pristiurus_-embryo in which the rudimentary external gills are present as very small knobs. Camera. (Zeiss CC, ocul. 2.) The section shews the commencing differentiation of the posterior nerve-rudiment into root (_pr_), ganglion (_sp.g_), and nerve (_n_), and also the attachment of the nerve-root to the spinal cord (_x_). The variations in the size and shape of the cells in the different parts of the nerve-rudiment are completely lost in the figure. _pr._ posterior nerve-root. _sp.g._ ganglion of posterior root. _n._ nerve of posterior root. _x._ attachment of posterior root to spinal cord. _w._ white matter of spinal cord. _i._ mesoblastic investment to the spinal cord. Fig. H II. Section through the same embryo as H I. (Zeiss CC, ocul. 1.)