76 EVOLUTION AND DISEASE. 
with the intestine is pervious, it is not used for the trans- 
mission of yolk, for, except in the case of a few fish 
(fig. 36), the contents of the yolk-sac have not been 
detected in the alimentary canal. The yolk is taken up 
by the omphalo-mesenteric vessels circulating on the 
walls of the sac. If such is the 
case we must regard the vitello- 
intestinal duct as a vestigial struc- 
ture in the higher mammals, and 
as such it is a source of danger to 
the individual. In man the duct 
should normally disappear long 
before birth; not infrequently it 
persists and grows pari passu with 
the ileum to which it belongs. 
It may be ten centimetres in 
length, and remain adherent to 
the navel, or form a short, blunted 
outgrowth to the bowel. When 
it remains adherent to the navel, py¢. 36. — An elasmobranch 
a portion of small intestine may, sh recently hatched with its 
during any unusual movement of fee 
the viscera, become twisted over it, obstruction to the free 
passage of the contents of the bowel is established, and, 
unless quick relief is afforded by art, a fatal issue ensues. 
The vitello-intestinal duct leads to disastrous .conse- 
