32 THE STRUCTURE OF MAN 
agree in attributing to human embryos of 4 to 6 mm. an externally 
visible and segmented tail, with a nervous axis and a post-anal 
gut (cf. Fig. 208), in comparison with which the peculiar perma- 
nent internal tail of the adult is a very degenerate organ. In 
this early embryonic stage the tailconsists of only two or three 
segments, but at a later period there are six caudal segments, the 
terminal mesodermal mass being reckoned merely as one. At 
this stage the tail consists of a number of segments, which are 
but very rarely retained permanently or even for a long time. 
The post-anal gut seems to be constricted off from the cloaca 
at this stage, but it 1s continued for the greater part of its course 
along the whole length of the embryonic tail. It apparently 
reaches its maximum length at this period (cf. Fig. 20, al”). 
At a later stage of development also, when thirty-six 
somites or body segments are formed, the post-anal gut can still 
be traced, but is no longer tubular. The caudal region at this 
stage possesses four spinal gangla with three related nerves. At 
a later stage the post-anal gut degenerates altogether. 
To sum up, we have the following purely anatomical facts 
which indicate that Man’s ancestors possessed a tail :— 
(1) The coccyx of the adult consisting of three to six caudal 
vertebree. 
(2) The two caudal spinal nerves. 
(3) The caudal musculature, the existence of which, further, 
is a direct proof that the tail was external and func- 
tional (cf. p. 277). 
(4) The vortex coccygeus and the foveola and glabella 
coccygea (cf. p. 5). 
(5) The variability of the caudal region in general. 
The other divisions of the human vertebral column also 
furnish many interesting points. One of the most characteristic 
peculiarities of the human backbone is its typical mode of curva- 
ture. The lumbar portion (cf. Fig. 23, B), which extends to the 
promontory of the sacrum and is convex anteriorly, deserves 
special attention. This lumbar curvature might appear to owe 
its origin to statical and mechanical causes connected with the 
upright gait, but while it is less markedly developed in the 
anthropoid Apes, [it has been shown by Cunningham and Charpy 
to be at least anticipated in certain quadrupedal Mammals].* 
1 [Huxley was the first to appreciate the existence of the lumbar curvature in the 
anthropoid apes, and Cunningham, Turner, and Symington have more recently drawn 
