GROWTH OF THE FETAL CENTRAL NERVOUS SYSTEM 451 
The length of the colliculi is not a true index of the mid- 
brain growth in early fetal life, and its significance as a measure 
of growth of that portion of the mesencephalon below the iter 
is particularly questionable. In some fetuses of three to four 
months a sharp line of demarcation between the superior col- 
liculi and the thalamencephalon may be present, while in other 
specimens of the same age no line of separation can be distin- 
guished. 
This conclusion is supported by evidence shown in figures 36, 
a,b, ec. Figures 36, a, and 36, b, of specimens prior to the third 
fetal month show no colliculi; figure 36, c, shows colliculi well 
marked off and relatively larger than in a new-born specimen. 
The curve of the absolute length of the colliculi, when plotted 
against body length (fig. 19), isa straight line. It starts at 0.7 cm. 
at 15 em. (C H) and rises to 1.22 cm. in length at 50 cm. 
(C H). It may be expressed by the empirical formula: 
Length of colliculi (em.) = 0.015 C H body length (em.) 
+0.47 
The growth of the colliculi in absolute length when plotted 
against time (fig. 38, curve VI) approaches a straight line, al- 
though the rate of growth is more rapid in the fourth and fifth 
months than in the last four fetal months. 
_ Growth of the spinal cord 
The growth of the spinal cord resembles that of the brain and 
particularly that of the pons and medulla. The absolute volume 
of the spinal cord plotted against crown-heel length (fig. 21) is a 
very shallow but typical volume curve, which may be expressed 
by the empirical formula: 
Volume of the spinal cord (ec.) = 0.01 [(0.17 C H 
length (em.)) 2:57 + 11.0] 
The absolute volume at 15 em. (C H) is approximately 0.22 ce. 
The curve rises steadily until it reaches about 1 ec. at 35em. (CH). 
From this point it proceeds upward at a sharper pitch, reaching 
about 2.5 cc. at 50 em. (C H). 
The growth in absolute volume against time (fig. 35, curve IV) 
corresponds almost exactly with that of the midbrain and shows 
