THE NERVOUS SYSTEM. 



527 



The early histological development of the cerebellum has been most closely 

 studied in Bony Fishes (Schaper) and there is every reason to suppose that 

 the processes taking place in the human cerebellum are essentially the same. In 

 that part of the alar plate forming the rudiment above described, the cells pro- 

 liferate, forming first a nuclear layer with the dividing cells along its ventricular 

 surface, and a non-nucleated outer or marginal layer. Later, owing to begin- 

 ning migration and differentiation, there is formed the usual mantle layer, 

 representing a differentiation of part of the original nuclear layer and thereby 

 forming the three layers: an inner, a mantle and a marginal. The outer cells 

 of the mantle layer increase in size and differentiate into the cells of Purkinje, 

 snaller cells within forming the granular layer. The earliest stage of differ- 

 entiation of the Purkinje cells has not been accurately described, but the axones 



FIG. 458. Diagram representing the differentiation and migration of the cerebellar cells in a teleost. 

 The arrows indicate the migration of cells from the borders of the cerebellar rudiment into 

 the marginal layer; these cells probably all differentiate into nerve cells. Clear circles, indif- 

 ferent cells; circles ivith dots, neuroglia cells (except in marginal layer); shaded cells, epithelial 

 cells; circles with crosses, epithelial cells in mitosis (germinal cells) ; black cells, neuroblasts; L 

 lateral recess; A/, median furrow, above which is roof plate; R, floor of 4th ventricle (IV). 

 Schaper. 



of the neuroblasts evidently proceed (end of fifth month) toward the ventricular 

 surface instead of entering the marginal layer. In this way the fibrous layer 

 (white matter) comes to lie within instead of on the outer surface as in the cord, 

 and, to some extent, in the medulla. There is thus formed the outer gray matter 

 or cortex. The axones of the Purkinje cells form the great bulk of the centrifu- 

 gal fibers of the cerebellar cortex. The marginal layer becomes ultimately 

 the outer or molecular (plexiform) layer of the adult cerebellum. 



It has been seen that in the other parts of the tube development begins in 

 the medial parts of the lateral plates and thence advances toward their dorsal 

 borders, which actively develop after the corresponding stages have ceased in 

 the medial portions. The same is true of the cerebellar rudiment. In this, 

 the edges which border on the thin roof plate, i.e., those parts adjoining the 

 lateral recesses, the main roof of the fourth ventricle and the roof plate inter- 

 posed between the two original lateral cerebellar rudiments, are the last to pro- 



