FORMATION OF THE FILUM TERMINALE 5 
in its development than it was in the younger stage. Whereas in 
the 11-mm. embryo there existed a distinct elaboration- into 
ependymal, mantle and marginal zones, the mantle zone is com- 
pletely missing in the 30-mm. embryo, and we find thin walls 
consisting only of ependymal cells covered by a thin marginal 
zone. The coccygeal spinal cord in the 30-mm. embryo is in an 
earlier embryonic state than that of the 11.5-mm. embryo; that 
0080U) 
ve. 
11.5 mm (x25) 
30mm (x 12.5) 
Fig. 1 Profile reconstructions showing the spinal ganglia and their dorsal 
roots in the tail region of the human embryo. The last two lumbar ganglia are 
shown in white, the sacral ganglia are stippled, and the coccygeal ganglia are 
solid black. It will be noted that in the period included between these two 
stages marked regressive changes have affected the entire coccygeal region of the 
spinal cord, with complete disappearance of the last three coccygeal ganglia, in 
sharp contrast to the sacral region of the cord, which undergoes uninterrupted 
development. The reconstructions are taken from embryos No. 544, 11.5 mm. 
long, and No. 75, 30 mm. long, belonging to the Carnegie Collection. 
is, it has undergone dedifferentiation. In later stages the proc- 
ess goes still farther and, as has been pointed out by Kunitomo, 
this ependymal tube eventually becomes converted or redif- 
ferentiated into a fibrous strand. 
How much of the spinal cord is involved in this retrograde 
process can be seen by comparing the two stages shown in 
figure 1. In the region of the attachment of the fifth sacral 
nerve the wall of the cord remains thick and develops a well- 
