THE BRAIN. 405 



Anterior part of hypothal- 



amic region 

 Ventral Zone { , 



Tuber cmereum 



Infundibulum. 



( Falx cerebri ( 



, -. Part of chorioid tela of third -< Meninges. 

 Mesoblast / t . , / 



v. ventricle. v. 



The telencephalon is at first a single vesicle forming the fore 

 part of the anterior primary vesicle, and for a time is open in front 

 at the neuropore (Fig. 118, VI). The median portion of its an- 

 terior wall, lamina terminalis, remains almost stationary; laterally, 

 it is the seat of rapid growth. 



Optic Vesicle. Almost before the telencephalon is differen- 

 tiated from diencephalon, a club-shaped diverticulum, called the 

 optic vesicle, is thrown out from the ventral part of its dorsal 

 zone. It grows outward and forward, separates from the telen- 

 cephalon and, becoming indented, forms the optic cup, from which 

 the retina is developed. The point of origin of the optic vesicle 

 is indicated in the adult by a slight pit, termed the optic recess. 

 The optic recess is situated between the lamina terminalis and the 

 columna fornicis at the anterior end of the sulcus hypothalamicus 

 (Monroi). This sulcus, in the anterior primary vesicle, sepa- 

 rates the ventral from the dorsal zone. 



Hemisphere of Cerebrum (Figs. 17, 118 and 120). Soon 

 after the appearance of the optic vesicle, a large bulging occurs on 

 either side in the dorsal zone of the telencephalon. That bulging 

 produces a hollow diverticulum, whose cavity is the primitive 

 lateral ventricle and whose walls form the substance of the hemis- 

 phere. The outgrowth is called the hemisphere vesicle. Its 

 constricted stalk contains the primitive interventricular foramen 

 (Monroi, Fig. 17). The vesicles grow forward and outward at 

 first, separated from one another by mesoblast which forms the 

 falx. Later, growth occurs in succession, upward, backward and 

 downward, until by the seventh month the hemisphere overhangs 

 every other part of the brain. The hemisphere vesicle is formed 

 almost wholly by the dorsal lamina of the telencephalon which 

 grows and develops wonderfully and will be considered further 



