. Chordate anatomy. Chordata; Anatomy, Comparative. REPRODUCTION 63 into a deep position, its conversion into a tube, and the covering of it by lateral ectoderm take place simultaneously (Fig. 54). Not until the tubular form is attained does the neural ectoderm of vertebrates become detached from the overlying superficial ectoderm. Figure 55 shows, in a diagrammatic way, the characteristic appearance of a recently formed neural tube with its neural crests, dorso-lateral extensions of ectodermal material on each side of the tube. Later the neural crest becomes detached from the tube, undergoes
. Chordate anatomy. Chordata; Anatomy, Comparative. REPRODUCTION 63 into a deep position, its conversion into a tube, and the covering of it by lateral ectoderm take place simultaneously (Fig. 54). Not until the tubular form is attained does the neural ectoderm of vertebrates become detached from the overlying superficial ectoderm. Figure 55 shows, in a diagrammatic way, the characteristic appearance of a recently formed neural tube with its neural crests, dorso-lateral extensions of ectodermal material on each side of the tube. Later the neural crest becomes detached from the tube, undergoes segmentation corre- sponding to that of the myotomes, and gives rise to spinal ganglia (Fig. 345). Cells of the crest become ganglion cells whence grow out nerve fibers which constitute the dorsal sensory root of a spinal nerve. The fibers of the other Fig. 55.âStereogram of embryonic constituent root of a spinal nerve, the "''^'â ^} ^^^^ showing the segmenting neural crest, e, superficial ectoderm; ventral motor root, grow out from cells nc, neural crest; s, central canal. within the neural tube. Some cells of (^^°"^ Kingsley, " Comparative Anat- omy of ;) the neural crests migrate mto various visceral localities and give rise to ganglia ("sympathetic"; Fig. 345) and nerves of the autonomic system. The anterior region of the tube expands to form the brain. Three enlargements, the primary brain vesiclesâfore-brain, mid-brain and hind-brain (Figs. 57, 58)âcharacterize the cephalic part of the tube in all vertebrate embryos. Later subdivision of the first and third vesicles results in the five brain regions universally characteristic of adult verte- brates. The nervous structures (retina and optic nerve) of the paired eye grow out from the second (numbered from the front) region but the lens of the eye is derived from neighboring superficial ectoderm (Fig. 56). The receptor (that is, stimulus-receiving) nervous structures of the ea
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