376 THE DEVELOPMENT OF THE CHICK 



hence to the vena cava inferior. With the degeneration of the 

 mesonephros, the subcardinals disappear in large part and t^e 

 postcardinals 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 venae cavse 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 com.ing 

 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 



