50(, 



NERVOUS SYSTEM 



seen, in all casL-s Ibrmed frqm the ectotK'nii (pp. 202, 201), 

 and Figs, (14, 65, and 143), and in craniate Vcitebratcs the 

 anterior end of the hollow jiieduUary tube becomes dilated, 

 lorniinn" three bulb-like swellings — the fore-brain (Fig. 

 14S, A, / /'), mid-brain (///. /'), and hind-hrain (//. />). Soon 

 a hollow oiitpushing grows forwards from the first \esicle 



TnatZ 



Vw.. 147. — 'l"r:ins\cr-.c ^ci. Imii of emijryo of froi;. 

 lot. cirjtoimj ; cut', proloiiyiition of cnjloiiie into proto\CTtft)r.i (tile i-efereiice liiiL- 

 sliould end .a tlit sp.-ti.c-) ; cut. me^eiltei'oil (:irclit;nten:iii) ; jiisd. mesoderm ; ucli. 

 iiorouliord : pf,'\ piuloverteljr.-i ; s^.tt. pronephric '^\\<:\\ som. soni.itic l.-iyer 

 of mesoderm; sp.c. >pi[i.-il cord; s/>t. spl.irichiiic l.'i\er of mesoderm; \li. 

 yolk-cells, (i'rom Piirker and H.xsu'ell's ZooAyr, aftci .M.-irsh.ill.) 



(1!, /«. eii), and the third gives off a similar hollow out- 

 growth {(/iltii) from its dorsal surface. The brain now con- 

 sists of five divisions: the prDseiiceplialoit {prs. m) and 

 (Uciicephahiii (die 11) deri\ed from the fore-brain, with the 

 pineal apparatus (///. /', ///. c) and the infundibulum 

 and pituitary body (/;// pty) : the itiid-hrain or iiiescn- 

 ctphaloii [ill. h) which gi\es rise to the optic iolvs and t:nti(i 

 (cn-bn : an(J the epciiciplialoii or ccrel>ellitiii (clihii) and 



