THE BRAIN 



335 



and constitutes the anterior medullary velum of the adult. Caudally, the ependy- 

 mal 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. 320). 



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 character- 

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

 completed at or after birth. 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. 



Alar plate 



Marginallayer 

 '. Nucleus N. Ill 



Root fibers N. Ill 



Fig. 321. — Transverse sections through the mesencephalon of a 10.6 mm. embryo. A, through the 

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

 (His). D. IV, decussation of oculomotor nerve; M.I., mantle layer; Nu. IV, nucleus of oculomotor 

 nerve. 



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

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

 nucleus may be 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. (For a detailed account of the development of the cere- 

 bellum see Streeter, in Keibel and Mall, vol. 2, p. 67.) 



Mesencephalon. — The basal and alar plates can be recognized in this sub- 

 division of the brain and each differentiates into the three primitive layers (Fig. 

 321). In the basal plate the neuroblasts give rise to the axons of motor nerves, 

 the oculomotor cranial, the trochlear caudal in position (Fig. 321 B). In ad- 

 dition to these nuclei of origin, the nucleus ruber (red nucleus) is developed in 



