Hamitton, Division of Cells in Nervous System. 303 
development, before the blood vessels have formed. He be- 
lieves this to be true of all hollow organs lined with epithelium; 
cell division is at first most active on the surface lining the 
cavity, both because here the resistance is lessened and because, 
in all probability, the fluid in the cavity serves as nutrition for 
the new cells. Later on, however, when the development of 
the blood vessels has taken place and the nutrition is equalized, 
the extra-ventricular mitoses appear. 
Paton (7), working onthe brain of pig embryos, finds 
very few extra-ventricular mitoses and considers their occasional 
appearance a phenomenon of little importance. 
The contradictory results obtained by these different au- 
thors may perhaps best be explained as RauBER explained 
them, by the fact that they were obtained from embryos repre- 
senting widely different stages of development. It may be that 
those who found cell division confined to the ventricular surfaces 
were studying much younger than those who found mitoses 
scattered through the gray and white matter. It is difficult to 
determine corresponding stages of development in different 
animals when only measurements are given, especially as in 
some instances the fresh, in others the hardened embryo is 
measured, but drawings of the cross sections of brain and cord 
make it possible to compare the stages with some degree of 
accuracy. Merk’s drawings for instance, are most of them 
from quite early embryos, the germinating layer is very wide, 
the nerve cells immature, and therefore it is not remarkable that 
extra-ventricular mitoses were very few. One drawing of the 
cord, however (Fig. 5) and one of the brain (Fig. 2) shows a 
stage of development which is represented by the foetal rat 
in my series. In MrErx’s drawings the mitoses are seen to be 
confined to the ventricular surfaces—in the embryo rat in my 
series the ventricular mitoses predominate, but each section 
contains two or three dividing cells in the outer layers. Another 
drawing in Merx’s article is from the cord of a 7% day chick 
(Fig. 10) when the stage of development is about midway be- 
tween the foetal and new-born rat in my series. Even here 
no extra-ventricular mitoses are shown. 
