566 EZRA ALLEN 
firms the observations of Fragnito and Capobianco with regard 
to the mesoblastic source: 
From these observations the conclusion is drawn that the neuroglia 
nuclei in the white rat as well as in the mouse represent two distinctly 
characterized types: namely, nuclei the structure of which resembles 
very closely that of the nerve cells, and the nuclei the structure of which 
resembles very closely that of endothelial cells which form the capillary 
wall. These two types of nuclei have been derived from the ectoblast 
and mesoblast respectively. The latter type has probably two sources 
of origin; that is, they are partly derived from mesoblastic cells immigrat- 
ing from the meninges (Capobianco and Fragnito), and partly from 
proliferating endothelial cells of the walls of the capillaries, these cells 
having separated from the capillary wall and migrated into the surround- 
ing tissue, where they constitute one type of the neuroglia elements. 
Hamilton (01) differentiates two types of dividing cells in the 
cerebrum and spinal cord, the one, large with cytoplasm well 
developed and the chromosomes more scattered, the other small 
without cytoplasm and with the chromosomes solidly bunched. 
The former, she thinks, develop into nerve cells and the latter 
into neuroglia. Hardesty (04) is of the opinion that the neuroglia 
cells cannot be differentiated from the inward-migrating white 
fibrous corpuscles. My observations lead me to agree with 
Hardesty, and so far as my methods of staining reveal any differ- 
entiations there seems to be no way of determining whether the 
small dividing cells are to become neurones, neuroglia cells or 
white fibrous corpuscles, since the small neurones which show 
processes very plainly are no larger than some of the dividing 
cells (figs. 14 and 16). 
My preparations indicate that division in the two sizes of 
mitotic cells does not continue to the same period in the cord and 
cerebrum. In the cord, the large cells are found dividing in 
various regions up to the sixth day (fig. 13). While twelve-day 
specimens still show some mitoses in the germinal zone, the divid- 
ing cells of the extra-ependymal regions possess the characteris- 
tics of the smaller type. In the cerebrum the chromatic mass in 
the dividing cell measures up to 7.5 x 8 micra in the younger 
material, while in 70-day and 120-day material, a list of measure- 
ments runs as follows, the tissue from the two animals having 
