Fig. A. Section through the dorsal region of an embryo of _Scyllium
stellare_, with the rudiments of two visceral clefts. The section
illustrates the general features at a period anterior to the appearance
of the posterior nerve-roots.
_nc._ neural canal. _mp._ muscle-plate. _ch._ notochord. _x._
subnotochordal rod. _ao._ rudiment of dorsal aorta. _so._ somatopleure.
_sp._ splanchnopleure. _al._ alimentary tract. All the parts of the
section except the spinal cord are drawn somewhat diagrammatically.
Figs. B I, B II, B III. Three sections of a _Pristiurus_-embryo. B I is
through the heart, B II through the anterior part of the dorsal region,
and B III through a point slightly behind this. Drawn with a camera.
(Zeiss CC, ocul. 2.)
In B III there is visible a slight proliferation of cells from the
dorsal summit of the neural canal.
In B II this proliferation definitely constitutes two club-shaped masses
of cells (_pr_), both attached to the dorsal summit of the neural canal.
The masses are the rudiments of the posterior nerve-roots.
In B I the rudiments of the posterior roots are of considerable length.
_pr._ rudiment of posterior roots. _nc._ neural canal. _mp._
muscle-plate. _ch._ notochord. _x._ subnotochordal rod. _ao._ dorsal
aorta. _so._ somatopleure. _sp._ splanchnopleure. _al._ alimentary
canal. _ht._ heart.
Fig. C. Section from a _Pristiurus_-embryo, slightly older than B.
Camera. (Zeiss CC, ocul. 2.) The embryo from which this figure was taken
was slightly distorted in the process of removal from the blastoderm.
_vr._ rudiment of vertebral body. Other reference letters as in previous
figures.
Fig. D a. Section through the dorsal region of a _Torpedo_-embryo with
three visceral clefts. (Zeiss CC, ocul. 2.) The section shews the
formation of the dorsal nerve-rudiments (_pr_) and of a ventral anterior
nerve-rudiment (_ar_), which at this early stage is not distinctly
cellular.
_ar._ rudiment of an anterior nerve-root. _y._ cells left behind on the
separation of the external skin from the spinal cord. _c._
connective-tissue cells springing from the summit of the muscle-plates.
Other reference letters as above.
Fig. D b. Section from dorsal region of a _Torpedo_-embryo somewhat
older than D a. Camera. (Zeiss CC, ocul. 2.) The posterior
nerve-rudiment is considerably longer than in fig. D a, and its pedicle
of attachment to the spinal cord is thinner. The anterior
nerve-rudiment, of which only the edge is present in the section, is
distinctly cellular.
_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.