?66 NERVOUS SYSTEM CH. XI 



seen, in all cases formed from the ectoderm (pp. 202, 209, 

 and Figs. 64, 65, and 143),' and in craniate Vertebrates the 

 anterior end of the hollow medullary tube becomes dilated, 

 forming three bulb-like' swellings — the fore-brain (Fig. 

 148, A,/ h), mid-brain {m. b), and hind-brain {h. b). Soon 

 a hollow outpushing grows forwards from the first vesicle 



jnsd 



Fig. 147. — Transverse section of embryo of frog. 

 Cixt. ccelome ; cxi', prolongation of coslome into protovertebra (the reference line 

 should end at the space) ; ent. mesenteron (archenteron) ; jnsd. mesoderm ; nch. 

 notochord ; pr. v. protovertebra ; s£: d. pronephric duct ; som. somatic l.tyer 

 of mesoderm ; sp. c. spinal cord ; spl. splanchilic layer of mesoderm ; yk, 

 yolk-cells. (From Parker and Haswell's Zoology^ after Marshall.) 



(B, prs. en), and the third gives off a similar hollow out- 

 growth (cblni) from its dorsal surface. The brain now con- 

 sists of five divisions : the prosencephalon {prs. en) and 

 diencephalon (dien) derived from the fore-brain, with the 

 pineal apparatus {pn. b, pn. e) and the infundibulum 

 and pituitary body (inf. pty) : the mid-brain or mesen- 

 cephalon (ni. b) which gives rise to the optic lobes and crura 

 cerebri: and the epencephalon or cerebellum (cblm) and 



