376 THE DEVELOPMENT OF THE CHICK 
hence to the vena cava inferior. With the degeneration of the 
mesonephros, the subeardinals disappear in large part and the 
posteardinals then empty directly into the vena cava inferior 
by way of the renal veins, which have formed in the meantime. 
The embryonic renal portal system of birds is similar in all essen- 
tial respects to the permanent system of amphibia and consti- 
tutes a striking example of recapitulation. The left auricle of 
the heart receives the small pulmonary veins. 
Thus practically all of the blood is returned to the right auricle 
of the heart; a considerable part of it is diverted into the left 
auricle through the foramina in the septum atriorum, and thus 
the blood reaches both ventricles. Complete systems of valves 
prevent its regurgitation in any direction. 
It is an interesting question to what extent the different kinds 
of blood received by the right auricle remain separate and receive 
special distribution through the body. The blood poured in by 
the anterior ven cave is purely venous, and it seems probable 
from the arrangement of the sinus valves that it passes into the 
ventricle of the same side, and so into the pulmonary arch and 
through the ductus Botalli into the dorsal aorta, and thus in part 
at least to the allantois where it is oxygenated. The blood coming 
in through the posterior vena cava is purified and rich in nutrition, 
for part of it comes from the allantois, where it has been oxygen- 
ated, and part has passed through the renal portal circulation, 
where, no doubt, it has been purified of nitrogenous excretory 
matter, and the remainder is mostly from the yolk-sac and hence 
laden with nutrition. This blood appears to be diverted through 
the foramen of the septum atriorum into the left auricle, and 
thence to the left ventricle, and so out into the carotids and 
aortic arch. It would seem, therefore, to be reasonably certain 
that the carotids receive the purest and most nutritious blood, 
for the blood in the dorsal aorta is mixed with the blood from 
the right ventricle. There can be no reasonable doubt that the 
heart is a more effective organ for separate and effective distribu- 
tion of the various kinds of blood received by it than this account 
would indicate. But further investigation is necessary to deter- 
mine in what ways and to what extent this takes place. 
At the time of hatching the following changes take place: 
the umbilical arteries and vein are obliterated in the allantois, 
owing to drying up of the latter; their stems remaining as relatively 
