544 



COMPARATIVE ANATOMY 



bending is presumably the result of the elongation of the brain in limited 

 space, since the brain elongates more rapidly than does the head itself. 

 The cephalic flexure is well marked in the embryos of anamnia, but the 

 pontine and nuchal flexures scarcely appear. They become increasingly 

 evident as we pass from lower to higher amniotes. (Fig. 449) 



Brain Vesicles and Their Derivatives 

 (After Keibel and Mall) 



Telencephalon. The division of the primary forebrain into telen- 

 cephalon and diencephalon is effected by a dorsal constriction. The 

 hemispheres develop from the telencephalon by paired lateral expansions 

 of the alar plates. The roof plate takes no part in this, but persists 

 between the hemispheres as the lamina terminalis of the adult brain. 

 The paired hemispheric enlargements become conspicuous in a human 

 embryo of six weeks and by the fifth month, have overgrown the cere- 

 bellum. In a six weeks embryo two chief divisions appear in each hemi- 

 sphere, a dorsal expanded pallium and a ventro-lateral thickening, the 

 corpus striatum. The palHum is indistinctly divided into a more ventral 

 and median archipaUium, and a more dorsal neopallium. (Fig. 447) 



