THE BRAIN 



337 



cerebelli grows more rapidly than the deeper layers of the cerebellum, and 

 its principal lobes, folds, and fissures are formed (Fig. 337 C, D). The 

 hemispheres derived from the lateral lobes are the last to be differentiated. 

 Their fissures do not appear until the fifth month. 



Cranial to the cerebellum the wall of the neural tube remains thin 

 dorsally, and constitutes the anterior medullary velum of the adult (Fig. 338 

 B}. Caudally, the ependymal roof of the fourth ventricle becomes the 

 posterior medullary velum. The points of attachment of the vela remain 

 approximately fixed, while the cerebellar cortex grows enormously. As 

 a result, the vela are folded in under the expanding cerebellum (Fig. 338). 



The anlages of the cerebellum show at first differentiation into the same three layers 

 which are typical for the neural tube. During the second and third months, cells from the 

 ependymal, and perhaps from the mantle layer of the rhombic lip migrate to the surface 

 of the cerebellar cortex and give rise to the molecular and granular layers which are charac- 

 teristic of the adult cerebellar cortex (Schafer) . The later differentiation of the cortex is 

 only completed at birth, or later. The cells of the granular layer become unipolar by a 

 process of unilateral growth. The Purkinje cells differentiate later. Their axons, and 

 those of entering afferent fibers, form the deep medullary layer of the cerebellum. 



The cells of the mantle layer may take little part in the development of the cerebellar 

 cortex, but give rise to neuroglia cells and fibers and to the internal nuclei. Of these, the 

 dentate nucleus is seen at the end of the third month; later, its cellular layer becomes folded, 

 producing its characteristic convolutions. The fibers arising from its cells form the greater 

 part of the brachium conjunctivum. 



D. IV. 



Alar plate 



Marginal layer 



Nucleus 

 ' N. III. 



r....- R ot fibers N.I II. 



A B 



FIG. 339. Transverse sections through the mesencephalon of a- 10.2 mm. embryo (His). 

 A, Through the isthmus and origin of the trochlear nerve; B, through the nucleus of origin of the 

 oculomotor nerve. D. IV., Decussation of oculomotor nerve; M.I., mantle layer. 



The Mesencephalon. The basal and alar plates can be recognized in 

 this subdivision of the brain, and each differentiates into the three primi- 

 tive layers (Fig. 339). In the basal plate the neuroblasts give rise to the 

 axons of motor nerves the oculomotor cranial in position, the trochlear 



