GROWTH OF THE FETAL CENTRAL NERVOUS SYSTEM 483 
formulae (table 43). The growths of most of the group are ex- 
pressed by the general empirical formula: 
Y = (ax)* —e 
in which Y is the brain-part value in em., X is crown-heel length 
in em., and a, 6, and c are constants determined separately in 
each case for the frontal spinal length, spinal cord length, and 
the brain-cord length. 
Two other straight-line measurements, one of the colliculi 
length and the other of the pons length, do not follow the typical 
brain stem and spinal cord type of growth for some unknown 
reason. ‘They are straight-line curves and evidently express the 
growth of some specific factor rather than the growth of the 
brain stem of which they are a part. , 
The brain stem and cord growth are portrayed also in the 
curves of the spinal cord volume, the pons and medulla volume, 
and the midbrain volume. When calculated against crown-heel 
length (fig. 30, curves X, VIII, and XI, respectively), these 
volume curves show a relatively rapid absolute growth previous 
to the sixth month and a slow growth in the last four months of 
fetal life. They also approximate each other and fall definitely 
above the cerebral type of growth curve. They are practically 
identical when calculated against fetal months and reduced to a 
per cent of the new-born value (fig. 36, curves VI, IV and V, re- 
spectively). When the percentage increments of the spinal cord 
volume, the pons and medulla volume, and the midbrain volume 
are compared (either when caleulated for 5-em. C H intervals 
(fig. 26, curves X, VIII, and IX, respectively) or for age in fetal 
months (fig. 28, curves I, IJ, and III, respectively)), the distinctive 
types of the brain stem and spinal cord growthare observed. They 
show a slow rate of relative growth in the third month, which in- 
creases three-fold in rapidity in the fourth month. From this time 
until birth the growth rate decreases in rapidity so that these por- 
tions show a less rapid relative increase in the last three fetal 
months than does any other brain part. 
The classification is indicated also by volumetric formulae 
(table 48) which can be expressed by the general equation: 
Y = 0.01 [(aX)*+ ¢| 
