_ar._ Anterior root of a spinal nerve. _ch._ Notochord. _com._ Commissure
connecting the posterior roots of the spinal nerves. _i._ Mesoblastic
investment of spinal cord. _mp._ Muscle-plate. _n._ Spinal nerve. _nc._
Neural canal. _pr._ Posterior root of a spinal nerve. _spg._ Ganglion on
posterior root of spinal nerve. _v.r._ Vertebral rudiment. _w._ White
matter of spinal cord. _y._ Point where the spinal cord became segmented
off from the superjacent epiblast.
Figs. 1, 2, and 3. Three sections of a Pristiurus embryo belonging to stage
I. Fig. 1 passes through the heart, fig. 2 through the anterior part of the
dorsal region, fig. 3 through a point slightly behind this. (Zeiss CC,
ocul. 2.) In fig. 3 there is visible a slight proliferation of cells from
the dorsal summit of the neural canal. In fig. 2 this proliferation
definitely constitutes two club-shaped masses of cells (_pr_)--the
rudiments of the posterior nerve-roots,--both attached to the dorsal summit
of the spinal cord. In fig. 1 the rudiments of the posterior roots are of
considerable length.
Fig. 4. Section through the dorsal region of a Torpedo embryo slightly
older than stage I, with three visceral clefts. (Zeiss CC, ocul. 2.) The
section shews the formation of a pair of dorsal nerve-rudiments (_pr_) and
a ventral nerve-rudiment (_ar_). The latter is shewn in its youngest
condition, and is not distinctly cellular.
Fig. 5. Section through the dorsal region of a Torpedo embryo slightly
younger than stage K. (Zeiss CC, ocul. 2.) The connective-tissue cells are
omitted. The rudiment of the ganglion (_spg_) on the posterior root has
appeared, and the junction of posterior root with the cord is difficult to
detect. The anterior root forms an elongated cellular structure.
Fig. 6. Section through the dorsal region of a Pristiurus embryo of stage
K. (Zeiss CC, ocul. 2.) The section especially illustrates the attachment
of the posterior root to the spinal cord.
Fig. 7. Section through the same embryo as fig. 6. (Zeiss CC, ocul. 1.) The
section contains an anterior root, which takes its origin at a point
opposite the interval between two posterior roots.
Fig. 8. A series of posterior roots with their central ends united by a
dorsal commissure, from a longitudinal and vertical section of a Scyllium
embryo belonging to a stage intermediate between L and M. The embryo was
hardened in a mixture of osmic and chromic acids.
Fig. 9. The central end of a posterior nerve-root from the same embryo,
with the commissure springing out from it on either side.