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. G1, G2, G3. Three longitudinal and horizontal sections of an embryo somewhat older than F. The embryo from which these sections were taken was hardened in osmic acid, but the sections have been represented without tinting. G1 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.
Plate 23.
Fig. H I. Section through the dorsal region of a Pristiurus-embryo in which the rudimentary external gills are present as very small knobs. Camera. (Zeiss CC ocul. 2.)
The section shews the commencing differentiation of the posterior nerve-rudiment into root (pr), ganglion (sp.g), and nerve (n), and also the attachment of the nerve-root to the spinal cord (x). The variations in the size and shape of the cells in the different parts of the nerve-rudiment are completely lost in the figure.
pr. posterior nerve-root. sp.g. ganglion of posterior root. n. nerve of posterior root. x. attachment of posterior root to spinal cord. w. white matter of spinal cord. i. mesoblastic investment to the spinal cord.
Fig. H II. Section through the same embryo as H I. (Zeiss CC ocul. 1.)
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.