Herrick, Histogenesis of the Cerebellum. 69 



gration. Similar stages have been observed in other parts of the 

 brain, as in the optic lobes, 



A more detailed account of the fate of the superficial layer 

 is given in the supplement to Wood's Reference Handbook of 

 the Medical Sciences. From the article on Histogenesis of the 

 Nervous System, (p. 692) we quote the following paragraph : 

 "The ectal portions of the massive organs, especially the cere- 

 brum and cerebellum, become somewhat rapidly clothed with a 

 dense mass of granules which subsequently disappear (Fig. 496). 

 Professor His suggests that these are white blood-corpucles 

 which, after wandering from the sinuses surrounding the organ, 

 make a temporary halt near the surface before passing through- 

 out the substance of both gray and white matter. The period 

 at which they appear in the human embryo is about the end of 

 the second month. While not doubting the existence of amoe- 

 boid cells, as above described, the writer has suggested other 

 sources for part of these cells. It will be noted that the per- 

 ipheral zone of cells arises only after direct communication with 

 the ventricles has been cut off by interpolated white matter. 

 Subsequently the number of granules {e. g., beneath the Pur- 

 kinje layer in the cerebellum) rapidly increases. Their source 

 can only be the peripheral collection of granules. Fig. 496 

 shows that, at this stage, there is often rapid multiplication go- 

 ing on within that layer. Careful study has not revealed any 

 other source for these proliferating granules than diverticles of 

 the epithelium, from which migration in this more round-about 

 way is kept up. It is a familiar fact that the earlier granules 

 are derived from the proliferating neuroblasts of the ventricular 

 surface : it is inherently improbable that the subsequent ones 

 should spring from as distinct a source as the pia or its vessels. 

 The peripheral proliferating layer soon disappears by the migra- 

 tion of its elements toward the ventricle." 



One statement made by Schaper (p. 693) is in our belief 

 inapplicable to fishes or at least is not universal. He says, 

 " Das urspriingliche 'embryonale stutzgeriist ' geht bis auf die zu 

 eigentlichen ependymzellen sich wandelnden elemente mit aller 

 warscheinHchkeit friizeitig zu Grunde. " We have shown that 



