The section contains an anterior root, which takes its origin at a point opposite the interval between two posterior roots. The white matter has not been very satisfactorily represented by the artist. Figs. I I, I II. Two sections of a _Pristiurus_-embryo somewhat older than H. Camera. (Zeiss CC, ocul. 1.) The connective-tissue cells are omitted. Figs. I a, I b, I c. Three isolated cells from the ganglion of one of the posterior roots of the same embryo. Figs. K I, K II. Two horizontal longitudinal sections through an embryo in which the external gills have just appeared. K I is the most dorsal of the two sections. Camera. (Zeiss CC, ocul. 1.) The sections shew the relative positions of the anterior and posterior roots at different levels. _pr._ posterior nerve-rudiment. _ar._ anterior nerve-rudiment. _sp.c._ spinal cord. _n.c._ neural canal. _mp._ muscle-plate. _mp´._ first-formed muscles. Fig. L. Longitudinal and vertical section through the trunk of a _Scyllium_-embryo after the external gills have attained their full development. Camera. (Zeiss CC, ocul. 1.) The embryo was hardened in a mixture of chromic acid and osmic acid. The section shews the commissures which dorsally unite the posterior roots, and also the junction of the anterior and posterior roots. The commissures are unfortunately not represented in the figure with great accuracy; their outlines are in nature perfectly regular, and not, as in the figure, notched at the junctions of the cells composing them. Their cells are apparently more or less completely fused, and certainly not nearly so clearly marked as in the figure. The commissures stain very deeply with the mixture of osmic and chromic acid, and form one of the most conspicuous features in successful longitudinal sections of embryos so hardened. In sections hardened with chromic acid only they cannot be seen with the same facility. 036.png _sp. c._ spinal cord. _gr._ grey matter. _w._ white matter. _ar._ anterior root. _pr._ posterior root. _x._ commissure uniting the posterior roots. Figs. M I, M II. Two sections through the head of the same embryo as fig. B. M I, the foremost of the two, passes through the anterior part of the thickening of epiblast, which becomes involuted as the auditory vesicle. It contains the rudiment of the seventh nerve, VII. Camera. (Zeiss CC, ocul. 2.) VII. rudiment of seventh nerve. _au._ thickening of external epiblast, which becomes involuted as the auditory vesicle. _n.c._ neural canal. _ch._ notochord. _pp._ body-cavity in the head. _so._ somatopleure. _sp._ splanchnopleure. _al._ throat exhibiting an outgrowth to form the first visceral cleft. EXPLANATION OF PLATES 24, 25, 26. (XII. p. 549.) PLATE 24. LIST OF REFERENCE LETTERS.