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. Da, 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.