560 EZRA ALLEN 
The germinal layer is completed progressively in the different 
levels of the cord in the albino rat. The completion is earliest 
in the thoracic region, where it is fully formed by the twenty- 
day period. It matures next in the lumbar region, where the 
wall is closed in nearly every section of the twenty-five-day and 
in every section of the thirty-day-old animals. In the cervical 
region, finally, a few sections at the last-named age still show 
a nuclear-free myelospongium at the ventral portion. The 
dorsal end is closed at all levels in the interval between the six- 
day and twelve-day periods. 
2. Correlations in the cerebellum 
The most interesting feature here is the external granule 
layer. Mitosis continues in this layer after it has ceased in all 
other parts of this organ. It ceases when the inward migration 
of the nuclei has taken place, a movement which is usually com- 
pleted between the twentieth and twenty-fifth day (Addison 
11). I found that in one specimen examined the migration had 
been completed at twenty days. Addison (11) found in one of 
his preparations dividing cells in this layer at twenty-two days. 
Examination of two twenty-five-day specimens prepared by me 
revealed no mitoses; neither was any trace of the external gran- 
ule layer present. 
3. Correlations in the cerebrum 
The well developed and persistent mantle zone lying along the 
ectal wall of the lateral ventricles is of particular interest, for in 
it occur the dividing cells found in the oldest specimens which 
show mitoses, and this layer, unlike the external granule layer of 
the cerebellum, never entirely disappears in animals up to two 
years of age, the oldest which I have examined (fig. 9). The 
one-day and twenty-five-day specimens both show dividing cells 
at every level along the entire length of the ventricles. In each 
case they are most abundant at the level of the optic chiasma. 
The mantle zone, while in general following the outline of the 
ventricular wall, is not of the same thickness throughout in any 
