GROWTH OF THE FETAL CENTRAL NERVOUS SYSTEM 479 
temporal diameter are practically identical. They occupy the 
same characteristic intermediate position between the cerebellum 
and the brain stem and spinal cord absolute growth curves. The 
rate of growth of the fronto-occipital diameter, when calculated 
from 5-em. C H intervals (fig. 25, curves I and If) and also when 
calculated for age in fetal months (fig. 27, curves I and IJ), por- 
trays an intermediate percentage increment curve lying between 
the cerebellum above and the brain stem below. The formulae 
of the fronto-occipital length and the temporal diameter (table 
43) likewise show a characteristic grouping of the hemispheres to 
the general formula: Y = aX + b, in which Y is the fronto- 
occipital length or the temporal diameter in cm., X is the crown- 
heel length in em., and a and b are constants determined for the 
two formulae. 
Cerebral growth in volume shows a steady and relatively slow 
increase before the sixth fetal month and then a constant but more 
rapid increase from that time until birth. This is indicated by 
the right hemisphere volume, left hemisphere volume, and both 
hemisphere volume (figs. 4, 5, and 6, respectively). When cal- 
culated against crown-heel length and reduced to percentage 
basis (fig. 30), the right hemisphere volume, left hemisphere 
volume, and the volume of both hemispheres (curves IV, V, and 
VI, respectively), as well as the absolute curves of the central 
nervous system and the total brain volume (curves I and II, re- 
spectively), fall practically upon one another. The cerebellum 
volume curve lies below and the brain stem and spinal cord vol- 
ume curve above the curves dominated by the factor of cerebral 
growth. Upon inspection of the curve of both hemisphere 
volume when calculated against time in fetal months and reduced 
to a percentage basis (fig. 36, curve II), a similar central grouping 
between the cerebellum volume, the brain stem volume, and 
spinal cord volume is observed. ‘Turning to the figures upon the 
rate of growth of the cerebrum, which is determined for 5-cm. 
intervals (fig. 26, curves IV, V, and V1) and for age in fetal months 
(fig. 28, curve V), one observes the unswerving tendency of the 
cerebral growth to proceed faster before the sixth month and 
slower thereafter. 
