558 js EZRA ALLEN 
reason for this condition since at birth the cerebellum is devel- 
oped to only a slight degree, and its growth thereafter is very 
rapid during its first twenty-five days of life. 
Hamilton (’01) found that the rate of cell division is less at 
birth than before birth. Unfortunately her paper does not give 
the embryonic age at which the rate is highest. Otherwise we 
might fix approximately the two periods in the life history of the 
albino rat when this factor of growth is most important in the 
cord and cerebrum. Since the figures given by her are not in 
terms of number per cubic millimeter, we have not the data 
needed for an exact comparison of the two phases of growth. 
STRUCTURAL CORRELATIONS 
1. Correlations in the spinal cord 
If we compare sections of the cervical cord from one-day and 
twenty-day rats respectively, we see several morphological differ- 
ences (figs. 5 and 6). (1) In the first place, the wall about the 
central canal of the one-day animal shows germinal cells in mito- 
sis, and shows also that its myelospongium at each end lacks 
nuclei. Neither of these phenomena appears in the older material. 
In addition, the form of the canal in the older differs from that 
of the younger in that it approaches more nearly the strongly 
elongated oval assumed in the adult cord. (2) The number of 
cells in the immediate vicinity of the canal of the twenty-day 
animal is less than in the one-day old. (8) There is greater 
degree of maturity in all the cells of the older as indicated by 
their size and cytoplasmic development. And (4) the number 
of migrating cells is much less in the older than in the younger ani- 
mal, although the region of greatest migrating activity remains 
the same in both ages—the region just ventral to the canal. 
In most vertebrates the germinal and mantle layers about the 
canal have been completed some time before birth. One known 
exception is the chick. Merk (’86) figures the cord of a chick 
of seven days and five hours old which shows a nuclear-free 
space in the germinal zone at each end of the canal, a condition 
found in the albino rat at six days. 
