The whole of the nerves in question arise as outgrowths of a median ridge of cells, which makes its appearance on the dorsal side of the spinal cord ([fig. 264 A], pr). This ridge has been called by Marshall the neural crest. At each point, where a pair of nerves will be formed, two pear-shaped outgrowths project from it, one on each side; and apply themselves closely to the walls of the spinal cord ([fig. 264 B], pr). These outgrowths are the rudiments of the posterior nerves. While still remaining attached to the dorsal summit of the neural cord they grow to a considerable size ([fig. 264 B], pr).
Fig. 264 A. Transverse section through a pristiurus embryo shewing the proliferation of cells to form the neural crest.
pr. neural crest; nc. neural canal; ch. notochord; ao. aorta.
Fig. 264 B. Transverse section through the trunk of an embryo slightly older than fig. 28 E.
nc. neural canal; pr. posterior root of spinal nerve; x. subnotochordal rod; ao. aorta; sc. somatic mesoblast; sp. splanchnic mesoblast; mp. muscle-plate; mp´. portion of muscle-plate converted into muscle; Vv. portion of the vertebral plate which will give rise to the vertebral bodies; al. alimentary tract.
Fig. 265. Vertical longitudinal section through part of the trunk of a young Scyllium embryo.
com. commissure uniting the dorsal ends of the posterior nerve-roots; pr. ganglia of posterior roots; ar. anterior roots; st. segmental tubes; sd. segmental duct; g.e. epithelium lining the body cavity in the region of the future germinal ridge.