GROWTH OF THE FETAL CENTRAL NERVOUS SYSTEM 447 
75 per cent in the third month and descends steadily to approxi- 
mately 7 per cent in the tenth month. 
There is no doubt that the pons and medulla form a large part 
of the brain in early fetal life. Figures 38, a, b, ce, and d, sub- 
stantiate the percentage curve (fig. 18). They show that the 
pons and medulla form a very large portion of the entire brain 
volume at this time. By the middle of the third fetal month 
(fig. 38, b) fully one-third of the brain, as it is seen in mid- 
20 
cm 
0 5 10 15 £0 e5 30 35 AO A5 50 55cm. 
Fig. 16 Field graph and curve of the growth of the pons and medulla in fetal 
life, as shown by the pons length. Abscissa: total body length in em. Ordi- 
nate: pons length in cm. Individual cases indicated by solid dots. Curve 
drawn to the formula: Y = 0.0263X + 0.16. (Data from tables 25 and 26.) 
sagittal section, is formed by the pons and medulla. In the 
fourth and fifth months (figs. 38, b, e, d, and e) the pons and 
medulla become relatively smaller. 
Growth of the midbrain 
A consideration of the colliculi and the midbrain does not lead 
to quite such clean-cut results as does the study of the other 
brain parts. The midbrain, as it was prepared for the volume and 
weight determinations, included not only the colliculi above the 
iter, but also a portion of the brain stem below it. 
