7 6 E VOL UTION 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, ncT^-An eiasmobranch 

 a portion of small intestine may, fish recently hatched with its 



. . . r yolk-sac. 



during any unusual movement of 



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- 



