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.
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.
[3] The figures on these Plates give a fair general idea of the appearance presented by the developing spinal nerves; but the finer details of the original drawings have in several cases become lost in the process of copying.
The figures which are tinted represent sections of embryos hardened in osmic acid; those without colour sections of embryos hardened in chromic acid.
EXPLANATION OF THE PLATES [24], [25], [26]. (XII. p. 549.)
List of Reference Letters.