FORMATION OF THE FILUM TERMINALE 9 
over the first half of fetal life (twenty-five weeks). In the 
adult the corresponding points fall at the interval between the 
bodies of the first and second lumbar vertebrae. Thus in the 
first twenty-five weeks there is an ascent of the ventriculus 
terminalis from the level of the second coccygeal to the third 
lumbar vertebra, or a distance of nine segments, and there 
remain but two segments before the adult position is reached. 
One may say that the principal part of the migration is accom- 
plished during the first half of fetal life. 
The dura mater and its relations can be plainly recognized in 
the 67-mm. fetus, where it can be seen to reach and adhere to 
the filum terminale at the lower border of the fourth sacral 
vertebra, thereby sealing off the lower end of the subdural 
space. It is of interest to note that it undergoes very little 
change from its position here and that which it occupies in the 
adult. In the 111-mm. fetus it extends to about the same level 
and ends in the same manner. In the adult it terminates about 
two segments higher up. Thus the dural sac conforms more to 
its bony environment than does the spinal cord and shows very 
little of the migratory adjustment of position that is noted in 
the latter. We therefore find the ventriculus terminalis 
gradually receding cranialward from the caudal end of the sac. 
In figure 2 the specimens are enlarged upon a decreasing 
scale of magnification according to age, so that the segments of 
the different stages are brought to about the same size. This 
has been done in order to facilitate the comparison of segment 
levels. The actual elongation of the spinal root of a given nerve 
is greater, therefore, than would appear from the figure. 
Measurements of the dorsal root of the first sacral nerve from 
the margin of the ganglion to the point of entrance into the 
cord yield the following figures: 30-mm. fetus, 0.65 mm. long; 
67-mm. fetus, 4.75 mm. long; 111-mm. fetus, 12.25 mm. long; 
221-mm. fetus, 32 mm. long. 
The actual elongation of the first sacral root is indicated for 
the first three of these stages in figure 3, in which the topography 
of the spinal cord and the vertebral column is drawn on the same 
scale of enlargement. The dorsal root of the first sacral nerve 
