THE NERVOUS SYSTEM. 



499 



similar to the unipolarization of the cerebrospinal ganglion cell. The dendrites 

 begin to be formed during the migration, branch when the cell body reaches the 

 granular layer and there finally attain the adult form. Other undifferentiated 

 cells in the marginal layer send out horizontal processes the collaterals of which 

 envelop the Purkinje cell bodies, and form the baskets. The place vacated, so to 

 speak, by the migrating granules, is filled at the same time by the developing 

 dendrites of the Purkinje cells. These at first show no regularity of branching, 

 but subsequently differentiate into the definite branches of the adult condition, 

 at the same time advancing toward the periphery (Fig. 422). When they 



FIG. 422. Section through cerebellar cortex of a dog a few days after birth, showing the partial 

 development of the dendrites of two cells of Purkinje. Cajal. 



A, external limiting membrane; B, external (embryonic) granule layer; C, partly formed molecular 

 (plexiform) layer; D, granular layer; a, body of cell of Purkinje; &, its axone; c, and d, col- 

 laterals with terminal arborizations (e). 



reach this, the migration of the granules is completed and the molecular layer 

 is definitely formed. This condition, evidenced by the disappearance of the 

 outer granular layer, is usually reached in Mammals within two months after 

 birth, but in man not until the sixth or seventh year. There are observations 

 indicating that animals possessing completely developed powers of locomotion 

 and balancing at birth have more completely differentiated cerebella at that 

 time. The axones of the Purkinje cells form many embryonic collaterals which 

 are afterward reduced in number. 



Of the centripetal fibers to the cerebellum, those from the inferior olives 

 cross the median line of the medulla about the seventh or eighth week, and 

 thence advance to the vermis, reaching their final destination during the third 



