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.