3l6 COMPARATIVE MORPHOLOGY OF VERTEBRATES 



spondingly modified. The processes involved may be readily under- 

 stood from a comparison of figures 327 and 329. The yolk sac is to 

 be regarded as a diverticulum of the intestine while the allantois is a 

 similar outgrowth from the urinary bladder, itself a process of the ali- 

 mentary canal. These outgrowths naturally carry with them the 

 blood-vessels distributed to the parts from which they arise. Hence 

 the omphalomesenteric artery and the vitelline veins (derivatives of 

 the omphalomesenteric veins) extend out over the yolk, increasing in 

 number as well as in extent of their branches as the yolk sac spreads 

 over the yolk. 



In the same way the hypogastric arteries are carried out with the 

 allantois, these portions being called the allantoic or umbilical 

 arteries, the blood being carried back to the trunk by a single allan- 

 toic vein. These two kinds of vessels — arteries and veins — are con- 

 nected in the distal part of the allantois by a rich network of capillary 

 vessels. It is by these that the allantois is able (p. 285) to act in the 

 sauropsida as an organ of respiration. In the mammals, by means of 

 osmosis through the placenta, it is not only respiratory, exchanging 

 gases with the uterine walls (there is no exchange of blood with the 

 mother), but it serves as a recipient of nourishment by the passage 

 of plasma from the maternal tissues. 



From the foregoing statements it will be seen that in the saurop- 

 sida five vessels — three arteries and two veins — pass out through the 

 umbilicus to the foetal adnexa, but in the mammals, where the yolk is 

 wanting and the yolk sac reduced and transitory in character, the 

 omphalomesenteric artery and the vitelline vein disappear early, 

 leaving but three vessels in the umbilical cord. In the elasmo- 

 branchs, where there is a large yolk sac bu^ no allantois, only the 

 yolk sac (omphalomesenteric) circulation is found. 



Circulation in the Separate Classes 



CYCLOSTOMES present marked diflferences in the circulation of the two 

 groups, the petromyzons being nearly normal, the myxinoids decidedly aberrant. 

 The aortic arches vary in number with the number of gill pouches (p. 255). In 

 the myxinoids the common carotid is connected with all of the efferent branchials 

 by a trunk running parallel to the body axis, just dorsal to the gill pouches. The 

 intersegmental arteries of the dorsal region are irregular, sometimes alternating, 

 sometimes appearing in pairs on the two sides of the median line. In the 

 lampreys (fig. 335) the subcardinals are united behind, the postcardinals in 

 front, these latter uniting with the single inferior jugular of the left side to form 

 the unpaired Cuvierian duct, the presence of which renders the sinus venosus 



