GROWTH OF THE FETAL CENTRAL NERVOUS SYSTEM A85 
to the sixth fetal month and the tremendously rapid increment 
from that time until birth. When calculated against crown-heel 
length (fig. 30, curve VII), and also when calculated against time 
in fetal months (fig. 36, curve III), the distinct character of the 
cerebellum volume is apparent. The formula of the growth 
curve of the cerebellum (table 48) is also characteristic, although 
it falls with the same general formula as the brain stem and spinal- 
cord volume, viz.: 
Y = 0.01 [(aX)e + ¢] 
However, it will be noticed that the b factor is 4.9 in the case of 
the cerebellum volume. This means that while the spinal cord 
volume, midbrain volume, and the pons and medulla volume are 
increasing practically as the cube and therefore like typical 
volume curves, the cerebellum volume is growing at a rate 
approaching the ninth power of the body length. The percentage 
increment of the cerebellum volume both when calculated for 
5-em. C H intervals (fig. 26, curve VII) or for age in fetal months 
(fig. 28, curve IV), also demonstrates a specific type of growth 
and shows the great increment of the part in the later fetal 
months as indicated by other measurements of cerebellum 
growth. 
Compound growth. Compound growth indicates a summation 
of two or all types of growth in the central nervous system. It 
is typified by the volume and weight of the encephalon and the 
central nervous system volume. It is dominated by the growth 
of the cerebrum, since the cerebrum ranges between 85 and 95 
per cent of the encephalon throughout fetal life. The rates of 
growth of the compound growth curves, as shown by percentage 
increments calculated for 5-cm. intervals (figs. 26, curves I, II, 
and III) and for fetal months (fig. 28, curves VI, VII and VIII), 
approximate the growth rate of the cerebrum. However, it 
will be noticed that in the early fetal months the rate of growth 
is not quite so high, due to the slow development of the other 
brain parts at this period. 
The volumetric formulae (of the type here presented) of the 
central nervous system, the entire brain volume, and the en- 
