THE NERVOUS SYSTEM. 537 



month. The fibers from the pontile nuclei (middle peduncle) do not develop 

 until considerably later (end of the fourth month), the time of their reaching 

 their destination in the cerebellar hemispheres not being definitely known. 

 Many at least of the centripetal fibers do not reach their full development in 

 Mammals till birth or after. Some of these fibers {climbing fibers) form arbor- 

 izations around the inferior (axone) surface of the Purkinje cell bodies and 

 later creep upward, enveloping the upper surface instead, and finally the den- 

 dritic branches. Other centripetal fibers {mossy fibers) ramifying in the 

 granular layer are varicose fibers, at first otherwise smooth. From the vari- 

 cosities a number of branches are given off which later become abbreviated and 

 modified into the shorter processes of the adult condition. This final differ- 

 entiation occurs simultaneously with the final differentiation of the dendrites 

 of the granule cells with which they come into connection. The glia elements 

 apparently develop in a manner essentially similar to their development else- 

 where. 



The development of the internal nuclei of the cerebellum has not been 

 thoroughly investigated. The nucleus dentalus is well developed at the end 

 of the sixth fcetal month. Eminences passing forward and ventrally along 

 the sides of the isthmus are the earliest indications of the superior peduncles, 

 formed later by the axones of the cells of these nuclei. 



Corpora Quadrigemina. 



The mid-brain roof is an expansion of the alar plate of the mid-brain. 

 Later this differentiates into the anterior and posterior corpora quadrigemina. 

 In the former, by the usual ventricular mitoses (germinal cells), a nuclear 

 layer is formed with a non-nucleated marginal layer external to it which becomes 

 the outer or zonal layer. Still later the neuroblast or mantle layer is differen- 

 tiated, there being an unusually thick inner layer. The further development 

 has not been closely studied in man. Owing to the diminished importance 

 of the anterior corpora quadrigemina (p. 474) the neuroblasts do not differ- 

 entiate into the well marked "spread out" layers characteristic of the optic 

 lobes of many Vertebrates. This is probably due to a lack of development of 

 their association neurones. 



The fibers of the optic tracts grow toward the anterior corpora quadrigemina 

 in the marginal layer forming the anterior brachia. When they reach the 

 anterior corpora quadrigemina, they leave the marginal layer and penetrate 

 the gray matter forming the most external fiber layer. The medial (and some 

 lateral) lemniscus fibers enter more deeply than the optic. Neuroblast axones 

 grow toward the ventricle, turn internally to the lemniscus fibers, cross {Mey- 

 nerl's decussation), and proceed as the predorsal tracts to the segmental brain 

 and cord, lying ventral to the medial longitudinal fasciculi. 



