GROWTH OF THE FETAL CENTRAL NERVOUS SYSTEM 489 
relative growth of these parts. The percentages which the brain 
stem and the spinal cord form of the encephalon are relatively 
high from the second to the fifth fetal month; the per cent which 
the cerebellum form of the encephalon is at its height in the last 
three fetal months, while the per cent which the hemispheres form 
of the encephalon reaches its maximum in the sixth fetal month. 
V. The four subtypes of growth of the central nervous system 
may be clearly expressed by a classification of the empirical for- 
mulae of the growth curves of the various parts as follows: 
1. Cerebral growth: 
a. General linear formula: Y = aX + 6. 
b. Specific linear formulae: 
a’. Fronto-occipital diameter: Y = 0.175 X + 0.25. 
b’. Temporal diameter: Y = 0.188 X + 0.3 
c. General volumetric formula: Y = (aX). 
d. Specific volumetric formulae: 
a’. Right hemisphere volume: Y = (0.1 X)*. 
b’. Left hemisphere volume: Y = (0.105 X)*-%. 
c’. Both hemisphere volume: Y = (0.12 X)*1°. 
2. Brain stem and cord growth: 
a. General linear formula: Y = (aX) — ¢. 
b. Specific linear formulae: 
a’. Fronto-spinal length: Y = (2.0 X)-47 — 2.2. 
b’. Spinal cord length: Y = (10.0 X)-4*7 — 4.0. 
c’. Brain cord length: Y = (10.0 X)-*4 — 4.0. 
c. General volumetric formula: Y = 0.01 [(aX)® + ¢]. 
d. Specific volumetric formulae: 
a’. Spinal cord. volume: Y = 0.01 [(0.17 -X)?- 
+ 11.0]. 
b’. Pons and medulla volume: Y = 0.01 [(0.2 X)?-57 
+ 20.0]. 
c’. Midbrain volume: Y = 0.01 (0. 168 X)2-55 + 12.0]. 
3. Cerebellum growth: 
a. General linear formula: Y = 0.01 [X¢ + bj. 
b. Specifie linear formulae: 
a’. Vermis cerebelli length: Y = 0.01 [X!-* + 0.15]. 
b’. Vermis cerebelli height: Y = 0.01 [X'-37]. 
