MITOSIS IN CENTRAL NERVOUS SYSTEM 563 
frontal section (figs. 7to 11). It is widest at the point where the ~ 
ectal lateral wall and the roof meet, and wider all along the ectal 
lateral wall than along the ental wall. In the one-day animal the 
layer extends all around the ventricle, but, beginning at the ven- 
tral portion, it is gradually reduced in extent until at twenty- 
five days old it consists of a single layer on the ental lateral wall; 
on the roof it is in almost the same condition, while on the dorsal 
portion of the ectal lateral wall and extending well outward at 
the point it is still several cells in thickness (fig. 10). Reduction 
is nearly complete along the ental lateral wall at the six-day period 
(fig. 8). In the twenty-five-day specimens cell division is occur- 
ring only in the portion of the ectal lateral wall and roof be- 
tween mn! and mn} in figure 10. In this region a total of eight 
dividing cells was found in a seventy-day animal in five frontal 
sections at the level of the optic chiasma, these being along the 
ectal lateral wall, with none on the roof. 
The mantle layer persists in this limited region along the ectal 
lateral wall in the two-year animal, as shown in figures 9 and 11. 
The active portion of this layer continues in advanced age to 
occupy the region a few cells in width external to the germinal 
layer, as shown by figures 21 and 22. There is evidently some 
connection between this layer and the “‘Uebergangsschichten” 
of His (’04) shown in his section of the human foetal cerebrum 
of four months. 
NATURE OF THE DIVIDING CELLS 
The tissues ultimately found in the nervous system are the 
neurones, the neuroglia and the connective tissue. In addition 
we find occasional leucocytes and a relatively greater number of 
lymphocytes. Hardesty (04) from his study of the developing 
neuroglia of the pig, concludes that, ‘‘With the present tech- 
nique there is nothing to show that all the products of the mitoses 
(germinal cells) in the ependymal layer are not indifferent ele- 
ments from the first—capable of developing into either neurones 
or neuroglia.” The neuroglia tissue has a double source of origin, 
arising from both the ectoblast and mesoblast. Hatai (’02) con- 
