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. 2 OB), in comparison with which the peculiar perma- 

 nent internal tail of the adult is a very degenerate organ. In 

 this early embryonic stage the tail consists of only two or three 

 segments, but at a later period there are six caudal segments, the 

 terminal mesoderrnal 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 is 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, oT). 



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 ganglia 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 

 vertebrae. 



(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. 27). 



(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 



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 



