Her RICK, Physiology of the Neri'ous Elements. 147 



referred to is peculiar to fishes and its late development 

 seems to indicate that this structure is a late acquisition 

 phylogenetically and this possibly indicates that the group 

 of fishes is a late off-shoot of more typical earlier (amphi- 

 bian-like) progenitors. Ascending the scale, we find that 

 the greater complexity of brain structure and (by corollary) 

 more extended development period introduced difficulties in 

 the way of carrying out of the simple plan of migration of 

 cells from the neuroblasts arising near the ventricle. That 

 such is their ultimate source cannot be doubted, but secon- 

 dary proliferating centres are necessary to adequately supply 

 with nerve cells those brain-areas which, as a result of the 

 remarkable folds of the originally simple brain tube come 

 to occupy regions far removed from the ventricle. Pro- 

 fessor His has shown that the olives and many other 

 structures of the medulla are formed by the separation of 

 the neuroblast cells from the ventricle and their transfer- 

 rence by means of folds and concretion of brain substance 

 to regions far removed from their source. The writer has 

 applied the same principle to the problem of cell-formation 

 in the cerebellum of certain mammals. It is also easily 

 observed in the development in the fish brain of the 

 hypoaria and nidulusniger. In the cerebrum of mammals 

 there is a curious temporary proliferating belt at the sur- 

 face which persists up to a comparatively late period of 

 embryonic life. 



It may be noticed that, although the active stage of 

 embryonic cell-proliferation is, in general, limited to a per- 

 iod more or less narrowly restricted, it does not by any 

 means follow that all the cells thus formed at once become 

 functional. In certain brain regions vast accumulations of 

 granules are formed which are essentially dormant nuclei 

 with the smallest possible amount of protoplasm. Such 

 nuclei are especially abundant in the cerebellum, optic 

 tectum, corpora striata and olfactory lobe, or, in general, 



