Reproduction 



273 



The anterior region of the tube 

 expands to form the brain. Three 

 enlargements, the primary brain 

 vesicles — forebrain, midbrain, 

 and hindbrain (Fig. 228) — charac- 

 terize the cephalic part of the tube 

 in all vertebrate embryos. Later sub- 

 division of the first and third vesicles 

 results in the five brain regions uni- 

 versally characteristic of adult verte- 

 brates. The nervous structures (re- 

 tina and optic nerve) of the paired 

 eye grow out from the second (num- 

 bered from the front) region, but 

 the lens of the eye is derived from 



Fig. 226. Stereogram of em- 

 bryonic neural tube showing the 

 segmenting neural crest, (e) Super- 

 ficial ectoderm; (nc) neural crest: 

 (s) central canal. (Courtesy, Kings- 

 ley: "Comparative Anatomy of 

 Vertebrates," Philadelphia, The 

 Blakiston Company.) 



Fig. 227. Stereogram of the 

 developing eye. The head of 

 the embryo is cut transversely 

 in the region of the forebrain. 

 (c/) Choroid fissure; (fb) wall of 

 forebrain; (/) ectodermal thicken- 

 ing which invaginates to form 

 lens; (oc) optic cup; {os) optic 

 stalk; (p) outer thin wall of optic 

 cup, becoming the pigmented 

 epithelium which lies behind the 

 definitive retina; (r) inner thick 

 wall of optic cup, becoming the 

 sensory retina of the eye. 

 (Courtesy, Kingsley: "Compara- 

 tive Anatomy of Vertebrates," 

 Philadelphia, The Blakiston 

 Company.) 



neighboring superficial ectoderm (Fig. 

 227). The receptor (that is, stimulus- 

 receiving) nervous structures of the ear 

 and olfactory organ originate not from 

 the neural tube but from superficial 

 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 ma- 

 terial is derived from middorsal en- 

 doderm and from the actively growing 

 region (germ-ring) about the blasto- 

 pore — the primitive streak of reptiles, 

 birds, and mammals being interpreted 

 as a modified blastoporal region. The 

 origin of the notochord is always closely 

 related to that of the mesoderm. In 

 reptiles, birds, and mammals, its ma- 

 terial, like that of the mesoderm, usu- 

 ally seems to be derived from the primi- 

 tive streak (see p. 263), a region where 

 ectoderm and endoderm merge indis- 

 tinguishably. As cells proliferated from 

 the streak laterally give rise to meso- 

 derm, so proliferation forward from the 

 anterior end of the streak produces a 



