THE BRAIN 337 



fused in the mid-line, producing a single structure marked by transverse fissures. 

 The rhombic lip gives rise to the flocculus and nodulus. Between the third and 

 fifth months the cortex 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 cere- 

 bellar 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 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 enter- 

 ing 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 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 cerebellum see Streeter, in Keibel and Mall, vol. 2). 



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. 

 339). In the basal plate the neuroblasts give rise to the axons of motor nerves — 

 the oculomotor cranial in position, the trochlear caudal (Fig. 339 B). In ad- 

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

 the basal plates ventral and somewhat cranial to the nucleus of the oculomotor 

 nerve. The origin of the cells forming the red nucleus is not definitely known. 

 The alar plates form the paired superior and inferior colliculi which together 

 constitute the corpora quadrigemina (Figs. 337 5 and 349). The plates thicken 

 and neuroblasts migrate to their surfaces, forming stratified ganglionic layers com- 

 parable to the cortical layers of the cerebellum and the cerebellar nuclei. With 

 the development of the superior and inferior colhculi the cavity of the mesen- 

 cephalic region decreases in size and becomes the cerebral aqueduct. 



