Hamitton, Division of Cells in Nervous System. 299 
TABEES i 
The Number of Mitoses in 15 consecutive Sections, each Section 6.75 wu in 
thickness, from the Brain and Spinal Cord of Rats at Different Stages of De- 
velopment. 
Stage of Brain. 
Development. Ventricular Mitoses. Extra-Ventricular Mitoses, 
Foetus 2196 966 
Birth 390 595 
24 hours 24 386 
4 days 115 443 
Lumbar Cord. 
Ventricular Mitoses. Extra-Ventricular Mitoses. 
Foetus 28 18 
Birth 8 45 
24 hours I 13 
4 days 8 64 
Cell division seems to reach its lowest ebb at the end of 
the first day. After this is slowly rises, until by the end of 
the fourth day, when the number of mitoses is not much behind 
that found at birth. 
These tables show clearly the actual diminution in the 
mitoses as development procecds, and the relative increase of 
the extra-ventricular mitoses over the ventricular. This relative 
increase reaches its height at the end of the first twenty-four 
hours, when the ventricular dividing cells are very few. After 
that there is an increase in mitoses in both regions, but slightly 
greater on the ventricular surfaces than in the outer layers. 
This relation of the numbers in the two regions may be seen in 
the following table : 
TABLE II. 
The Relative Number of Extra-Ventricular Mitoses Expressed as a Ratio. 
Based on the Observations on the Lumbar Cord given in Table I. 
Lumbar Cord. 
Ventricular. Extra-Ventricular. 
Foetus I to 0.6 
Birth I to 5-4 
24 hours I to 13. 
4 days I to 8. 
Although the actual number of mitotic figures varies ap- 
preciably in different animals from the same litter, which appar- 
ently are at constant stage of development, yet the proportion 
