ys GEORGE L. STREETER 
adjustments that occur in many regions in the development of 
the human embryo. 
Another instance of dedifferentiation has recently been pointed 
out by Kunitomo.? This writer has published the results of a 
careful study of the tail region in alargenumber of human embryos, 
representing the period of greatest development of the caudal 
appendage, and also the later period of its gradual reduction. 
He shows that in very young specimens the spinal cord reaches 
the extreme tip of the tail and throughout its length is quite 
uniform in structure. Somewhat later (11 to 15-mm. stage) it 
can be divided at about the level of the thirty-second vertebra 
into two parts—a cranial or main part, having a wide central canal 
and thick walls in which can be recognized well-developed mantle 
and marginal zones, and a caudal slender part, having a narrow 
canal with walls consisting only of an ependymal zone. Kunitomo 
shows that it is this caudal atrophic portion that eventually 
forms the filum terminale. The main part lying cranial to the 
thirty-second vertebra undergoes uninterrupted and progressive 
differentiation, whereas the portion caudal to this undergoes re- 
gressive changes and, with the exception of the extreme tip, 
finally becomes converted into a fibrous strand, the tip forming 
the coccygeal medullary vestige. This, therefore, is another 
instance in which an absorptive adjustment is brought about by 
the reversion of the tissue to an earlier embryonic type with a 
certain amount of subsequent redifferentiation. 
Kunitomo further calls attention to the fact that in the forma- 
tion of the filum terminale, in addition to the dedifferentiation 
of the caudal end of the medullary tube, there is also the 
mechanical disproportion between the growth of the medullary 
tube and that of the vertebral column. How much of one and 
how much of the other of these two factors is responsible for the 
further development of the filum terminale was not determined 
by him. It has occurred to the writer that this question could 
be answered by the determination of the elongation of the nerve 
2 Kunitomo, K., 1918. The development and reduction of the tail and of the 
caudal.end of the spinal cord in the human embryo. Contributions to Embry- 
ology, vol. 8, Publication No. 271, Carnegie Inst. of Wash. 
