426 EMBRYOLOGY. 



Histologically considered the wa'ils of the vesicles originally consist 

 everywhere of closely crowded spindle-shaped cells, just as in the 

 spinal cord. These cells undergo in different places unlike modifica- 

 tions. In some places they retain their epithelial character, and 

 furnish (1) the epithelial covering of the choroid plexus in the roof 

 of the between-brain and after-brain, (2) the ependyma lining the 

 ventricles of the brain, and (3) follicular structures such as the 

 epiphysis (fig. 246). On the greater part of the wall of the five 

 brain-vesicles the cells multiply to an extraordinary extent, and are 

 transformed into more or less extensive layers of ganglionic cells and 

 nerve-fibres. The distribution of the gray and white substances thus 

 formed no longer presents in the brain-vesicles the same uniform 

 condition that it does in the spinal cord. The only uniformity is 

 found in the fact that in every part of the brain there occur gray 

 " nuclei," which, like the anterior and posterior gray columns of the 

 spinal cord, are enveloped with a mantle of white substance. How- 

 ever, there are added to the two parts of the brain that have attained 

 the greatest development layers containing ganglionic cells, which 

 furnish a superficial covering, the gray cortex of the cerebrum and 

 cerebellum. By this means the white substance in certain parts of the 

 brain becomes the core (nucleus medullaris), whereas the gray portion 

 becomes the cortex, a condition differing in an important manner 

 from the structure of the spinal cord. 



The morphological differentiation of the brain depends upon the very 

 unequal growth both of the five separate vesicles and of different tracts 

 of their walls. For example, the other four vesicles remain in their 

 development far behind that of the cerebral vesicle, in comparison 

 with which they constitute only a small fraction of the entire mass of 

 the brain (figs. 240, 241). They become overgrown by the cerebral 

 vesicle from above and on the sides, and enveloped as by a mantle, 

 so that they remain uncovered and visible only at the base of the 

 brain. Therefore they, together with a small part of the basal 

 portion of the cerebrum, are grouped together as the stalk of the 

 brain, in contradistinction to the remaining chief part of the cere- 

 brum, which constitutes the cerebral mantle. 



The unequal growth of the walls of the brain manifests itself in the 

 appearance of thickened and attenuated places, in the development 

 of special nerve-columns (pedunculi cerebri, cerebelli, etc.), and in 

 the formation of more or less extensive layers of ganglionic cells 

 (thalamus opticus, corpus striatum). By these means the principle 

 of the formation of folds, which was fully described in the fourth 



