74 EVOLUTION AND: DISEASE. 
tooth ; this produces severe constitutional disturbance 
which may place life in jeopardy. 
When dealing with the intestinal origin of the spinal 
cord it was mentioned that the central canal of the cord 
and the gut communicated with each other around the 
caudal end of the notochord. This connecting passage 
is known as the neurenteric canal, and the section of the 
bowel into which it opens is known as the post-anal gut, 
because it is situated posterior to the permanent outlet 
of the bowel. In the elasmobranchs this section of the 
primitive gut equals a third of its total length. The_ 
coccygeal region is often the seat of congenital 
tumours, some of which present peculiar characters, 
The examples most interesting to us attain a very large 
size—often more than a kilogram in weight—and are 
situated anteriorly to the coccyx. Structurally they are 
composed of cysts lined with epithelium, the stroma 
consists of very young connective-tissue: sometimes 
these tumours contain a portion of bowel lined with 
mucous membrane, possessing Lieberkiihn’s follicles 
and Peyer’s patches. 
A study of the development, structure, and relations 
of the tumours will serve to convince any impartial 
observer that they arise in connection with the post- 
anal gut; they are by no means rare; few pathological 
museums of any pretensions are without a specimen, 
or a model of them, and all surgeons of experience 
have encountered one or more examples of them. The 
large specimens are incompatible with life, but smaller 
ones have been successfully dealt with surgically. 
Finally every gradation has been recorded, from per- 
