GROWTH OF THE FETAL CENTRAL NERVOUS SYSTEM 437 
2.3 cc. at six fetal months. At this time it begins to enlarge 
rapidly, increasing almost ten-fold in the last four months of 
intra-uterine life and reaching a volume of 21 cc. at birth. This 
tremendous growth in volume in the last three or four months of 
fetal life, at a time when all other brain parts are growing far less 
rapidly, is a peculiar characteristic of the cerebellum. 
The percentage increment of the cerebellum volume (fig. 26, 
curve VII) reflects the peculiarities of the curve of absolute 
volume. Starting at a comparatively low value, it rises to a 
peak of 112 per cent for the 25-to-30-cm. interval and descends 
to 60 per cent in the 50-to-55-cm. interval. 
The percentage increment of the cerebellum volume, when 
calculated for time (fig. 28, curve IV), shows an approximate rate 
of growth of about 160 per cent in the fifth month, and then falls 
rapidly to 54 per cent for the tenth month. 
The relative growth of the cerebellim is also illustrated by the 
per cent which it forms of the entire brain volume at various 
periods in fetal life (fig. 11). This value is, with slight variations, 
about 3 per cent until the middle of the sixth fetal month. At 
this time the tremendous growth of the cerebellum becomes an 
important factor in the total increase of the brain as a whole, and 
by birth it forms about 6 per cent of the entire brain volume. 
The absolute length of the vermis cerebelli, when plotted to 
crown-heel body length (fig. 12), is expressed by the formula: 
Vermis cerebelli length (cm.) = 0.01 [(C H length (cm.))'!.# 
+0.15] 
It also portrays the great growth of this part of the brain in the 
last four fetal months. At 10 em. (C H) the vermis cerebelli 
length is only about 0.4 em. It ascends in a very shallow curve 
40 2.6 em. at 50 em. (C Hy). 
The absolute length of the vermis cerebelli when calculated for 
fetal months (fig. 31, curve I) rises in practically a straight line 
from 0.4 cm. at the beginning of the fourth month to 2.64 cm. 
at birth. 
The rate of growth of the vermis cerebelli length (fig. 25, curve 
IIT) shows a gradual decrease from 50 per cent in the 10-to-15-cm. 
interval to 13 per cent at the 50-to-55-cm. interval. The entire 
