296 THE ('HICK. 



Shortly before the time of hatching, the allantoic ve 

 become constricted, by the closure of the body walls at the 

 umbilicus. The allantois itself shrivels up, and is cast off as 1 lie 

 chick works its wav out of the shell. 



DEVELOPMENT OF THE HEART AND BLOOD- 

 VESSELS. 



1. Preliminary Account. 



The general arrangement of the vascular system during 

 embryonic life is strikingly similar to that of the tadpole. The 

 heart is at first a straight, and later a twisted tube, lying beneath 

 the pharynx, and driving the blood through a series of paired 

 aortic arches (Fig. 128) to the dorsal aortse, which distribute it 

 to all parts of the body. From the body generally, and from the 

 Wolffian bodies, the blood is returned by anterior and posterior 

 cardinal veins on each side ; these unite to form the Cuvierian 

 veins, or anterior vense cavae, which open into the sinus venosus 

 or posterior end of the heart. From the alimentary canal the 

 blood is returned by the mesenteric or hepatic portal vein, which, 

 passing through the liver, joins the posterior vena cava, the vein 

 through which the blood is returned to the heart from the 

 kidneys and other organs. 



The chief differences between the chick and the frog as 

 regards the arrangement of the blood-vessels are : (i) that the 

 chick embryo has no gills, either external or internal, and there- 

 fore possesses no vessels corresponding to the gill loops of the 

 tadpole ; and (ii) that in the chick the vessels connected with the 

 yolk-sac and with the allantois, both of which are structures out- 

 side the embryo itself, are enormously developed. These blood- 

 vessels, vitelline and allantoic, are in direct connection with the 

 vessels of the embryo : the afferent vessels, i.e. the vitelline and 

 allantoic arteries, being branches of the dorsal aorta ; while the 

 efferent vessels, the vitelline and allantoic veins, on entering the 

 embryo, join the mesenteric veins and run, through the liver, to 

 the heart. 



Throughout the greater part of the period of incubation, the 

 vitelline and the allantoic vessels are of very large size ; and 

 inasmuch as the returning vessels, the vitelline and allantoic 

 veins, bring to the embryo food matter from the yolk-sac, and 



