8 GEORGE L. STREETER 
length of 100 to 200 mm. the ventriculus terminalis can be 
recognized in gross dissection with the naked eye, as a piriform, 
translucent area at the tip of the conus medullaris. Apparently 
in the natural condition it presents smooth outlines, but in pre- 
pared sections its thin walls are thrown into what are evidently 
shrinkage folds. This ventricle bears a permanent relation to 
the rootlets of the fifth sacral nerve at their entrance into the 
cord, as is shown in figure 2. In the four stages there represented 
it lies just caudal to the entrance of the fifth sacral nerve. The 
rootlets of the first coccygeal nerve in these specimens were so 
delicate that they could not be traced with certainty and were 
therefore omitted. The roots of the five sacral nerves, however, 
could be very accurately followed, and are all indicated in the 
figures. Their entrance into the substance of the cord con- 
stitutes in each case a firm attachment and remains a fixed topo- 
graphical point. By comparing the four stages from 30 mm. to 
221 mm. it will be seen that the ventriculus terminalis and the 
points of attachment of the sacral roots maintain the same 
relative positions, there being no further encroachment of the 
former into the territory of the more cephalic lying spinal cord. 
In other words, there is no further dedifferentiation of the sacral 
region of the cord after the embryo has attained a length of 30 
mm. The cephalic migration that is subsequently experienced 
by the ventriculus terminalis and points of attachment of the 
sacral nerve roots, relative to the bodies of the vertebrae, is 
clearly a result of the fact that the vertebral column gradually 
extends farther caudalward than the spinal cord, and since the 
nerve roots and the filum terminale are attached at both ends 
they are correspondingly elongated. The latter process is not a 
simple stretching, for, as these structures lengthen they actually 
become thicker. In other words, there is a compensatory inter- 
stitial growth. This increase in thickness is not apparent in 
figure 2, as the older stages are shown at a progressively de- 
creasing scale of enlargement. 
The rapidity and extent of the caudal thrust of the vertebral 
column—that is, its caudal displacement in relation to the 
terminal ventricle—can be seen in figure 2. This covers a little 
