22O THE EVOLUTION OF MAN. 



parts. Two of these contractions first appear, and con- 

 sequently the brain forms three consecutive bladders (Fig. 



3 * ' FIGS. 224-226. Central mar 



row of human embryo in the 

 seventh week, two cm. long. 

 (After Koelliker.) Fig. 226, 

 view of the whole embryo from 

 the dorsal side ; the brain and 

 dorsal marrow laid bare. Fig. 

 225, the brain and npper part 

 of the dorsal marrow from the 



left side. Fig. 224, the brain from above : v, fore-brain ; z, twixt-brain ; 



m mid-brain ; h, hind-brain ; n, after-brain. 



222, v, m, fi). The first and third of these three primitive 

 bladders then again separate by transverse contractions, 

 each into two parts, and thus five consecutive bladder-like 

 divisions are formed (Fig. 223 : cf. also Plate V. Figs. 

 13-16 ; Plates VI. and VII., second cross-line). These five 

 fundamental brain-bladders, which re-occur in the same form 

 in the embryos of all the Skulled Animals (Craniota), were 

 first clearly recognized by Baer, who understood their true 

 importance and distinguished them, according to their rela- 

 tive positions, by very appropriate names, which are still in 

 general use : I., fore-brain (v) ; II., twixt-brain (0); IIL, mid- 

 brain (m) ; IV., hind-brain (A) , and V., after-brain (n). 



In all Skulled Animals, from the Cyclostomi to Man, 

 the same parts, although in very various forms, develop 

 from these five original brain-bladders. The first bladder, 

 the fore-brain (protopsyche, v), forms by far the largest part 

 of the so-called " great brain " (cerebrum) ; it forms the two 

 great hemispheres, the olfactory lobes, the streaked bodies 

 (corpora striata), and the cross-piece (corpus callosum), 

 together with the " arch " (fornix). From the second 



