THE MESENCEPHALON. 1105 



of the sulcus primarius — itself the X^X^x preclival fissure — separate the anterior and posterior 

 crescentic lobules. During the fourth month the postlimate fissure appears, in each hemisphere, 

 on the upper surface of the posterior lobe. By the extension and medial union of these sulci, for 

 a time separate, are established the posterior limit of the clivus {postclival fissure) and the 

 demarcation between the posterior crescentic and the postero-superior lobule. The post-tonsillar 

 fissure bounds the conspicuous elevation of the tonsil behind and medially joins the infrapyram- 

 idal (later prepyramidal ) sulcus. The parapyramidal fissure defines the upper (posterior) 

 limit of the biventral lobule and unites with the suprapyramidal (later postpyramidal) fissure. 

 The great horizonial fissure, so conspicuous in the mature cerebellum, appears relatively late, 

 about the end of the fifth month, and is at first represented by a shallow transverse median fur- 

 row that lies immediately in front of the suprapyramidal fissure (Bolk) , an origin at variance 

 with the generally accepted formation of the horizontal fissure by the union of two lateral sulci, 

 that grow medially from the hemispheres and meet in the worm. Ths early fissure having such 

 history, Bolk identifies as the postlunate (sulcus superior posterior) and not as the horizontal. 

 This author also emphasizes the fact that at the sixth fcetal month the folium cacuminis is, as a 

 rule, not only defined, but forms a well-marked superficial tract that connects the adjoining, 

 lateral tracts (postero-superior lobules). This part of the worm, however, does not keep pace 

 with the cortical expansion of the surrounding parts and, hence, becomes overgrown by these 

 and sinks into the relative insignificance that distinguishes this part of the worm in the fully 

 matured cerebellum. In consequence of the rapid growth and expansion of the peripheral 

 portions of the human cerebellum, some fissures of secondary morphological importance, as 

 the horizontal, become excessively deepened and more conspicuous in man than those of 

 fundamental significance, as the sulcus primarius (preclival) and the postnodular fissures. This 

 cortical expansion, especially within the superior region, likewise brings about prominent 

 changes in the position of the segments of the worm, so that eventually those which primarily 

 lay behind later come to lie below, the divisions of the conventional upper and lower worm of 

 the mature cerebellum following along the C-like curve seen in sagittal sections. 



The histogenesis of the cerebellar cortex probably primarily proceeds from the invasion of 

 the cellular lamina by the cells of the dorsal zones of the lateral walls of the metencephalon, as 

 well as directly from these zones themselves. The earliest differentiation results in the production 

 of three strata : {a) the \rm.&c ependymal layer, and {b) the middle ^waw/Zi? layer, and (r) the outer 

 marginal layer. Of these the mantle layer is the thickest and richest in cells, from which both 

 neuroblasts and spongioblasts arise, although their differentiation occurs relatively late. The 

 Purkitije cells, early distinguishable by their large clear nuclei, appear during the sixth foetal 

 month, but for some time lack their characteristic processes. Likewise from the mantle layer 

 are derived the earliest constituents of the granule layer. Meanwhile within the marginal layer, 

 immediately beneath the external surface of the cerebellum, an additional and temporarily con- 

 spicuous cell-stratum, the external granule layer, becomes a prominent feature of the develop- 

 ing cerebellar cortex. This layer soon exhibits a subdivision into two zones of which the outer 

 contains many dividing cells, while the inner is almost free from karyokinetic figures. During 

 the later months of fcetal life the inner sublayer disappears and at birth the outer one is greatly 

 reduced ; finally, this also disappears, so that after the earliest years of childhood the external 

 granule layer is no longer seen. The chief factor in this reduction and eventual obliteration 

 of this stratum is, according to Cajal, the gradual transformation of its neuroblasts into nerve- 

 cells that recede from their peripheral position to assist in the completion of the granule layer, 

 as whose small and characteristically branched elements they persist. Other neurones of the 

 external granule layer are transformed into the basket cells and the large stellate cells. The 

 neuroglia of the cerebellar cortex is derived chiefly from the spongioblastic elements of the inner 

 or ependymal layer, the conversion of the cells of the outer granule layer into the supporting 

 tissue, as sometimes assumed, being unlikely (Ziehen). Since the molecular layer is composed 

 to a considerable extent of the dendritic processes of the Purkinje cells, the development of the 

 outer division of the cerebellar cortex is complete only after the growth of such processes, as 

 well as of the climbing fibres from tlie white core, has taken place. 



The production of the superior cerebellar peduncles and of the definite superior medullary 

 velum is dependent upon the development of the fibres that pass from and to the dentate 

 nucleus and the cerebellar cortex — an invasion that occurs during late foetal and early post- 

 natal life. 



THE MESENCEPHALON. 



Notwithstanding its considerable size and prominent positisn in the embryo, in 

 its mature condition the mesencephalon, or mid-brain, forms the smallest and least con- 

 spicuous division not only of the brain-stem but also of the entire brain. Neverthe- 

 less, the many fundamental tracts which it contains, as well as the new paths and 

 combinations which arise within its substance, confer on the mid-brain an importance 



70 



