FORMATION OF THE FILUM TERMINALE Lt 
is indicated by a heavy black line; the first thoracic, first 
lumbar, first sacral, and first coccygeal vertebrae are marked 
by small circles. Comparison of the stages, as shown in this 
figure, gives perhaps a better representation of the actual topo- 
graphical changes that occur in this apparent ascent than does 
figure 2. 
From these results we may conclude that in the human 
embryo the greater part of the coccygeal and post-coccygeal 
cord—that is, the part caudal to the thirtieth segment—under- 
goes dedifferentiation, the more cephalic part of it persisting as 
the ventriculus terminalis and the more caudal part redif- 
ferentiating into a fibrous strand—the fi!um terminale, with the 
coccygeal medullary vestige at the tip. The first twenty-nine - 
segments of the spinal cord are not affected by this process of 
dedifferentiation, but continue in a progressive development. 
When the embryo reaches 30 mm. in length there begins a dis- 
‘proportion in the rate of growth as between the vertebral column 
and the spinal cord, the former elongating more rapidly than 
the latter. This results in a relative displacement of the two, 
the ventriculus terminalis in the 221-mm. fetus (twenty-five 
weeks) lying nine segments higher than it did originally, and by 
the time the adult form is attained two more segments have been 
added to this disproportion. We may say, therefore, that the 
filum terminale represents that portion of the spinal cord caudal 
to the second coccygeal segment (thirty-first segment), which 
has undergone dedifferentiation and has finally become con- 
verted into a fibrous strand. This strand, like the sacral nerve 
roots, elongates by interstitial growth in adaptation to the 
ascending displacement of the spinal cord. The caudal tip of 
the dural sac maintains its relation to the vertebrae rather than 
to the spinal cord and remains attached to the filum terminale 
in the sacral region at a more or less fixed point. 
