CIRCULATORY ORGANS. 293 
left atrial portion of the heart, dividing farther back to reach the two 
lungs. At no time do the pulmonary veins connect with the sinus 
venosus, but they always empty into the left atrium (fig. 285). 
The Fetal Circulation. 
Some features of the foetal circulation of the amniotes have already 
been alluded to, but the whole may be summarized here. In 
the amniotes, with the development of a large yolk sac and of the 
allantois, the vessels on the ventral side of the body become corre- 
spondingly modified. The processes involved may be readily under- 
stood from a comparison of figs. 282 and 284. 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 ever 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. 264) 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 they serve as recipients of nourishment by the passage 
of plasma from the maternal tissues. 
From the foregoing statements it will be seen that in the sauropsida 
five vessels—three arteries and two veins—pass out through the um- 
bilicus 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, leav- 
ing but three vessels in the umbilical cord. In the elasmobranchs, 
where there is a large yolk sac but no allantois, only the yolk sac cir- 
culation is found. 
