THE NERVOUS SYSTEM. 



535 



liferate. The cells thus formed spread into the marginal layer of the earlier 

 developed parts and by further proliferation form a nucleated layer of consider- 

 able thickness (Fig. 458). This complication is apparently essentially similar 

 to that described above in the development of the medulla. From the cells of 

 this invasion are formed a part, at least, of the granule cells, as well as the basket 

 cells and other cells which remain in the marginal (molecular) layer. These 

 are all association cells of the cerebellum. 



The cerebellum reaches its full histological development very late; after 

 birth in many Mammals. These last postnatal stages of development naturally 



D 



K 



FIG. 459. Scheme showing the various stages of position and form in the differentiation of granule 

 cells from the outer granular layer. Cajal. 



A, Layer of undifferentiated cells; B, layer of cells in horizontal bipolar stage; C, partly formed 

 molecular (plexiform) layer; D, granular layer; b, beginning differentiation of granule cells; 

 c, cells in monopolar stage; d, cells in bipolar stage; e,f, beginning of descending dendrite 

 and of unipolarization of cell; g,h,i, different stages of unipolarization or formation of single 

 process connecting with the original two processes; ;, cell showing differentiating and com- 

 pleted dendrites; k, fully formed granule cell. 



involve principally those cells proliferated last and which lie in the mar- 

 ginal layer. These have been studied by means of the Golgi method in 

 new-born Mammals by Cajal and others. The majority of these cells form 

 granule cells by means of a progressive migration and differentiation, as shown 

 in the accompanying Fig. 459. Each cell first develops a single horizontal 

 process, then another, thus becoming a horizontal bipolar cell. Following this, 

 the cell body migrates past the Purkinje cells into the granular layer, remaining 

 in connection with the original processes by a single process. There are thus 

 formed the axone of the granule cell with its bifurcation into two horizontal pro- 

 cesses, the parallel fibers of the molecular layer. This mode of formation is thus 



