750 



AVES. 



Fig. 377. 



derm has spread extensively over the yelk (fig. 375, b) ; and as the 

 vessels formed by it become perfected, they are found to converge to the 

 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 foetus, and forming a vascular circle surrounding the yelk. 

 The omphalo-mesenteric arteries (fig. 375, 6, 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 supe- 

 rior vena cava of the young 

 chick. 



(2137.) As soon as the in- 

 testinal system of the embryo 

 bird is distinctly formed, the 

 membrane enclosing the yelk 

 (vitellicle) is seen to commu- 

 nicate with the intestine by 

 a wide duct (ductus vitello- 

 intestinalis), whereby the nu- 

 tritive substance of the yelk 

 enters the alimentary canal 

 to serve as food, and the 

 mucous membrane lining the 

 vitellicle becomes thrown into 

 close wavy folds, so as to pre- 

 sent a very extensive surface. 

 Gradually, as growth ad- 

 vances, the yelk diminishes 

 in size ; and at length, before 

 the young bird is hatched, 

 the remains of it are entirely 

 withdrawn into the abdominal 

 cavity (figs. 378, 379), 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. 



(2138.) While the above phenomena are in progress, another im- 

 portant system of vessels, provided for the respiration of the bird in ovo, 

 is developed, and obliterated before the egg is hatched. 



(2139.) At ^ about the period represented in fig. 374, the sides of the 

 abdominal cavity, which is still open anteriorly, are occupied by transi- 

 tory secreting organs, named corpora Wolfiana ; these, apparently, are 

 the rudiments of the genito-urinary system ; and, to receive their secrc- 



Ves8els of the allantoi^. 



