BRAIN 



201 



embryonic epithelial character : this is usually regarded as being 

 due to regressive metamorphosis, the cause of which, however, is 

 difficult to explain. The relative distribution of the gray and white 

 matter differs in various parts of the brain. 



Connecting the two lateral halves of the brain are certain 

 transverse bands of nerve-fibres or commissures. In addition to a 

 small superior or habenular commissure in the pallium (Fig. 150), 

 an anterior commissure is present in the posterior region of the 

 secondary fore-brain, a middle in the primary fore-brain (in 

 Mammals only), and a posterior in the anterior part of the mid- 

 brain. In addition to these, others may be developed in the pallial 

 region (e.g. anterior and posterior pallial commissures, cf. Fig. 165) ; 

 and amongst Mammals those known as the corpus callosum and 

 fornix are of great importance. 



Diencepha/.on 



FIG. 150. DIAGRAMMATIC LONGITUDINAL SECTION THROUGH PART OF THE 



EMBRYONIC BRAIN. 



Zirbdpolster, pineal cushion. 



The outer surface of the hemispheres is more or less smooth, 

 except amongst the Mammalia, in which fissures (sulci) and convolu- 

 tions (gyri) may be present. These consist of folds of the entire 

 pallium or cortex, and they cause a greater or less increase of the 

 superficial area. 



From the primary fore-brain, the ventricle of which is walled 

 in anteriorly by the lamina terminals, the following structures also 

 arise (Fig. 149) : the optic thalami, formed as thickenings of its 

 basal walls, and the ganglia hdbenulcc on the posterior lateral 

 margin of the dorsal region, with the superior commissure between 

 them ; the primary optic vesicles, arising as paired ventro-lateral 

 outgrowths, from which the optic nerves and retina with its pigment 

 epithelium are derived later ; the pineal apparatus, developed as 



