

DEVELOPMENT OF THE NERVOUS SYSTEM 



743 



mass of the ventricle is developed across the area of contact. The metathalamus 

 comprises the geniculate bodies which originate as slight outward bulgings of the 

 alar lamina. In the adult the lateral geniculate body appears as an eminence on 

 the lateral part of the posterior end of the thalamus, while the medial is situated 

 on the lateral aspect of the mid-brain. The epithalamus includes the pineal 

 body, the posterior commissure, and the trigonum habenulse. The pineal body 

 arises as an upward the evagination of roof-plate immediately in front of the mid- 

 brain; this evagination becomes solid with the exception of its proximal part, 

 which persists as the recessus pinealis. In lizards the pineal evagination is elongated 

 into a stalk, and its peripheral extremity is expanded into a vesicle, in which a 

 rudimentary lens and retina are formed; the stalk becomes solid and nerve fibers 

 make their appearance in it, so that in these animals the pineal body forms a 

 rudimentary eye. The posterior commissure is formed by the ingrowth of fibers 

 into the depression behind and below the pineal evagination, and the trigonum 

 habenulse is developed in front of the pineal recess. 



Choroidal fissure 





Rhinencephalon 



Lamina terminalis 



Corpus striatum 



Optic recess 

 Chiasma 



Hypophysis 

 Recessus infundibuli 



FIG. 654. Median sagittal section of brain of human embryo of three months. (From model by His.) 



From the basal lamina? of the diencephalon the pars mamillaris hypothalami 

 is developed; this comprises the corpora mamillaria and the posterior part of 

 the tuber cinereum. The corpora mamillaria arise as a single thickening, 

 which becomes divided into two by a median furrow during the third month. 



The roof-plate of the diencephalon, in front of the pineal body, remains thin and 

 epithelial in character, and is subsequently invaginated by the choroid plexuses 

 of the third ventricle. 



The Telencephalon. This consists of a median portion and two lateral diver- 

 ticula. The median portion forms the anterior part of the cavity of the third 

 ventricle, and is closed below and in front by the lamina terminalis. The lateral 

 diverticula consist of outward pouchings of the alar laminae; the cavities represent 

 the lateral ventricles, and their walls become thickened to form the nervous 





