6 GEORGE L. STREETER 
differentiated mantle zone. About opposite the first coccygeal 
nerve in the 30-mm. embryo the mantle zone abruptly disappears, 
and there is a corresponding enlargement of the lumen of the ~ 
cord, thereby producing the thin-walled ventriculus terminalis. 
There is some variation in different embryos as to the segmental 
level caudal to which the mantle zone has dedifferentiated and 
also in the manner of transition from the well-developed sacral 
cord into the atrophic coccygeal cord, including sometimes the 
doubling or partial obliteration of the central canal. The tran- 
sition is quite abrupt, involving only one segment. In the 
30-mm. embryo in figure 1 the cord at the level of the first 
coccygeal nerve shows some decrease in the size of its mantle- 
zone area. Opposite the second coccygeal nerve the mantle 
zone is entirely gone. The second coccygeal ganglion present 
in this case would probably soon have disappeared. 
The ventriculus terminalis at this stage tapers caudally and 
may be said to extend to the third coccygeal segment. Caudal to 
this the differentiation of the cord is more complete and results 
in the gradual obliteration of the lumen and the replacement of 
the ependymal substance by a fibrous strand, embedded in 
which can be found isolated groups of persistent ependymal 
cells. At its extreme tip there is often found a more or less 
detached group of such cells which undergoes cystic enlargement 
and constitutes the coccygeal medullary vestige. The interval 
of cord lying between this and the ventriculus terminalis con- 
stitutes what is later known as the filum terminale. Thus far 
its formation is based upon the process of dedifferentiation; its 
subsequent growth and elongation is accomplished by an inter- 
stitial increase of its constituent fibres, and not by the further 
invasion of the process of dedifferentiation into the sacral region 
of the cord. -This will become evident on examination of 
figure 2 
It has been pointed out that in embryos 30 mm. long a ven- 
triculus terminalis is formed opposite the second and third 
coccygeal vertebrae, owing to a retrogressive thinning out of 
the walls of the spinal cord, with a consequent irregular enlarge- 
ment of the central canal. In fetuses with a crown-rump 
