THE NERVOUS SYSTEM. 549 



is continuous with the thalamus (peduncular part) , may be considered as a fixed 

 point from which the pallial walls expand in all directions, anteriorly, dorsally 

 and posteriorly, i.e., in both transverse and longitudinal directions. At first, 

 this expansion causes the pallial hemispheres to assume a bean-shape with the 

 hilum at the fixed point (Fig. 465). The anterior end curves downward and 

 forms the frontal lobe with its enclosed cavity {anterior horn of the lateral ven- 

 tricle). The posterior end curves downward caudally and forms the temporal 

 lobe with the descending horn of the lateral ventricle. At the same time, owing 

 to the great expansion in a transverse plane of each pallial eminence, the 

 median lamina uniting them (Figs. 463 and 464) not sharing in this growth, 

 there are formed the hemispheres with their cavities, the lateral ventricles, and 

 the great longitudinal fissure between the hemispheres. Later, . vascular 

 mesodermal tissue fills this fissure, forming the falx cerebri. The paired 

 cavities of the pallium are connected with the unpaired end-brain cavity (aula) 

 by the foramina of Monro, the boundaries of which are the same as those of the 

 pallium described above (p. 545). 



At first the walls of the telencephalon, like those of other parts of the tube, 

 are epithelial in character and nearly uniform in thickness. By proliferation 

 there is formed a several-layered epithelium differentiated into an inner 

 nuclear layer and an outer marginal layer. Later a mantle layer is differen- 

 tiated. The hemispheres are late in development and until the end of the 

 second month the walls are thin and simply show the above three layers. 

 Toward the end of the first month a greater activity in cell proliferation takes 

 place in the basal portion of the telencephalon which thickens into the corpus 

 striatum. At eight weeks there first appears on the external surface of the 

 corpus striatum, a cortical layer of cells lying next the marginal layer and sepa- 

 rated from the inner layer by an intermediate layer comparatively free of cells 

 and known as the fibrous or medullary layer (see p. 561). The differentiation 

 thus begun extends gradually around the circumference of the hemispheres 

 until the mesial surface is reached. This differentiation permanently ceases 

 at the medial pallial margin. The cortical layer does not extend as far as 

 the medullary layer, thus leaving an uncovered medullary layer on the mesial 

 hemisphere wall. As a result of this, there is in this region, passing toward 

 the median line, (1) a region covered with a cortical layer {limbus corticalis 

 of His) ; (2) an uncovered medullary layer {limbus medullaris); (3) a fibrous 

 transitional zone (the tcenia) passing into (4) a membranous zone, the roof 

 plate. 



This process resembles that taking place in other parts of the neural tube, 

 in which there is the same progressive development from the ventral portion 

 of the lateral wall to the dorsal border of the same, where the latter passes into 

 the roof plate which is either ependymal or expanded into a thin membrane. 



