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 Ea. 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.)