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.— Stereogram of embryonic 



constituent root of a spinal nerve, the "''^'■^} ^^^^ 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 Vertebrates.") 



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 vesicles — fore-brain, mid-brain and 

 hind-brain (Figs. 57, 58) — characterize 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 ear 

 and olfactory organ originate not from the neural tube but from super- 

 ficial ectoderm. 



Notochord. The notochord in the several classes of vertebrates 

 exhibits many variations in details of its mode of origin. The essential 

 fact is that, in vertebrates as in Amphioxus, its material is derived from 

 mid-dorsal endoderm and from the actively growing region about the 

 blastopore. In amniotes the origin of the notochord is closely related 

 to that of the mesoderm. Its material, like that of the mesoderm, 

 usually seems to be derived from the primitive streak (see page 54), 

 a region where ectoderm and endoderm merge indistinguishably. As 

 cells proliferated from the streak laterally give rise to mesoderm, so 

 proliferation forward from the anterior end of the streak produces a 

 median cord of cells which form the notochord. It may, however, receive 



