THE NERVOUS SYSTEM. 



461 



an evagination appears, the optic vesicle (Fig. 414) which develops into the 

 retina and optic nerve. 



In the next stage (Fig. 401), there is a tendency for the neural tube to bend 

 ventrally around the anterior end of the notochord. This bending is the 

 cephalic flexure. At the same time the dorsal wall above the cephalic fold, be- 

 comes expanded and is marked off from that part of the dorsal wall lying 

 caudally by a transverse constriction, the- rhombo-mesencephalic fold, and from 

 the part of the dorsal wall lying cranially by another transverse fold at the 

 site of the future posterior commissure. The middle part of the brain, the 

 roof of which is thus marked off, is the mid-brain or mesencephalon. Its 

 floor is the middle projecting part of the ventral cephalic fold. The cephalic 

 expansion of the brain, practically the former archencephalon, is now the 



Fig. 401. — Scheme of a median sagittal section through a vertebrate brain after the formation 

 of the three primary brain expansions, von Kupfjer. 

 P.. prosencephalon; M., mesencephalon; R., rhombencephalon; Ms., spinal cord; cw., chiasma emi- 

 nence; J., infundibulum; It., lamina terminalis; pv., ventral cephalic fold; pn., processus 

 neuroporicus; pr., rhombo-mesencephalic fold; r. 1 , unpaired olfactory placode; ro., recessus 

 (prae-?) opticus; tp., tuberculum posterius. 



fore-brain or prosencephalon and the caudal expansion is the rhombic brain or 

 rhombencephalon. 



These three primary brain expansions ("vesicles"), the fore-brain, mid- 

 brain and rhombic brain, are constant throughout the Vertebrates. Beginning 

 at the location of the former neuropore (processus neuroporicus) and passing 

 caudally along the floor of the fore-brain we have the lamina terminalis or end- 

 wall of the brain, containing a thickening which indicates the site of the future 

 anterior (cerebral) commissure, next the recessus prceopticus, then another thick- 

 ening, the chiasma eminence, and finally a diverticulum, the recessus postopticus 

 and infundibulum (Fig. 401). 



At a later stage (Fig. 402), there appear two evaginations in the roof of the 

 fore-brain, the anterior epiphysis or paraphysis and the posterior epiphysis or 

 epiphysis proper (pineal body). Immediately caudal to the paraphysis is a 

 transverse infolding of the brain roof, the velum transversum. The line aa 



