Herkick, J\forpho/ogv of I\'crvo/es Svstc//i. 9 



But the area of ventricular surface is progressively re- 

 stricted until it is reduced to insignificance, and can be 

 brought into no relation with the Deiters and Purkinje 

 cell-layers. His(') concludes that the nervous elements 

 have arisen by a subdivision of certain of the nuclei of the 

 epithelium near the ventricular wall and thence migrate to 

 their definitive foci. Such a direct migration is rendered 

 impossible in the present case by the early development of 

 the central tract of white matter. 



Again, it is a fact of observation that at a period not long 

 before birth in rodents, for example, the surface of the cere- 

 bellum ectad to the Purkinje cells, which are already pres- 

 ent, is covered by a thin zone of rapidly dividing cells, 

 which, as they increase, are also passing entad, to a level 

 beneath the cells of Purkinje. 



This remarkable assemblage of superficial cells with 

 dividing nuclei, caryokinetic figures and other evidences of 

 proliferation may be readily seen in the cerebellum of old 

 embryos of the guinea-pig (Plate II, Fig. 4, and Plate I, Fig. 

 9). The condition indicated in Fig. 9, where the corpuscles 

 are much more numerous superficially and are there more 

 frequently dividing than in the infraganglionic zone which 

 is their ultimate locus, and the fact that the superficial pro- 

 liferating zone almost entirely disappears in the adult suffi- 

 ciently shows that these cells are not indigenous, and sug- 

 gests the necessity of tracing the history of the proliferating 

 zone. That this zone does not arise in situ is rendered 

 somewhat probable by the fact that no such layer exists in 

 reptilia, but the ventricular surface is the proliferous locus, 

 even though in the higher reptiles it is reverted so as to 

 become spuriously ectal in position. On the other hand, it 

 would at first appear that we have a case quite comparable 

 to the present one in the cerebrum where the cells of the 

 cortex proliferate at the surface and migrate to their various 



I Wm. His, Archiv f. Anatoniie u. I'hysiologie, Anatomische Abtheilung, 1890, p. 95 

 et seq. 



