628 



AVES BIRDS. 



fig. 290. 



navel of the embryo, and to constitute a distinct system of arteries 

 and veins (omphalo-mesenteric), communicating with the aorta and 

 with the heart of the fetus, and forming a vascular circle surrounding 

 the yolk. The omphalo-mesenteric arteries, (Jig. 291, b, c,) which 

 thus ramify over the vitelline sac, are derived from the mesenteric 

 arteries ; and the blood distributed through them is returned, by the 

 omphalo-mesenteric veins, to the superior vena cava of the young 

 chick. 



(704.) As soon as the intestinal system of the embryo bird is 

 distinctly formed, the membrane enclosing the yolk (vitellicle) is 

 seen to communicate with the intestine by a wide duct (ductus 

 vitello-intestinalis), whereby 

 the nutritive substance of the 

 yolk enters the alimentary 

 canal to serve as food, and 

 the mucous membrane lin- 

 ing the vitellicle becomes 

 thrown into close wavy folds, 

 so as to present a very ex- 

 tensive surface. Gradually, as 

 growth advances, the yolk di- 

 minishes in size ; and at length, 

 before the young bird is hatch- 

 ed, the remains of it are entirely 

 withdrawn into the abdominal 

 cavity, (Jigs. 292, 293,) where 

 its absorption is completed : 

 but even in the adult bird 

 a little caecal appendage, or 

 diverticulum, still indicates 

 the place formerly occupied by the ductus vitello-intestinalis. 



(705.) While the above phenomena are in progress, another 

 important system of vessels provided for the respiration of the bird 

 in ovo are developed, and obliterated before the egg is hatched. 



At about the period represented in Jig. 288, the sides of 

 the abdominal cavity, which is still open anteriorly, are occupied 

 by transitory secreting organs, named corpora Woifiana ; these, 

 apparently, are the rudiments of the genito-urinary system : and, 

 to receive their secretion, a bladder is developed, called the allan- 

 toid sac, a viscus which is moreover destined to play an impor- 

 tant part in the economy of the embryo, and soon becomes 



