FORMATION OF THE FILUM TERMINALE 3 
roots. In the younger stages the spinal cord and the vertebral 
column lie alongside of each other in a metameric manner, cor- 
responding in position segment for segment. Owing to their 
disproportion in growth, there occurs a relative displacement of 
their segment levels, so that, for instance, the thirtieth segment 
of the cord comes to lie opposite the twentieth segment of the 
vertebral column. The segment levels of the vertebral column 
are, of course, evident; in the spinal cord they are just as plainly 
marked by the attachment of the nerve roots, for these become 
attached to the cord before the displacement begins, and thus 
permanently mark the various segmental levels. In the case of 
each segment of the spinal cord there are two fixed topographical 
points: the spinal ganglion, which is held in the intervertebral 
foramen and registers the original position of the segment relative 
to the vertebral column, and the place at which the dorsal root 
is attached to the cord and which moves as the cord moves. By 
locating those points for the different stages one can determine 
the exact elongation of the nerve roots, and this in turn is the 
index of the relative displacement of the spinal cord as regards 
the vertebral column. Conversely, it will be seen that the 
alteration not explained by mechanical displacement must be 
attributed to the retrogressive changes referred to above. The 
determination of the amount of displacement was made by 
comparison of selected stages by means of profile reconstructions 
of the smaller specimens and actual dissection of the older ones. 
I was assisted in this by Mr. James F. Didusch, of the Carnegie 
Embryological Laboratory, who made careful dissections of these 
structures in several older fetuses, two of which will be used for 
illustration. The results of this determination are given in the 
following note as a matter of interest to those who have read 
the paper by Kunitomo, and also because it offers an opportunity 
to emphasize the significance of dedifferentiation of tissues in 
the processes of development in the human embryo. 
The part played by dedifferentiation in the caudal region of 
the spinal cord is more apparent in the younger stages of develop- 
ment, as pointed out by Kunitomo. The so-called ‘absorption’ 
of the tail is completed before the embryo reaches a length of 
