Development of the Fourth Day 259 



the heart: but, owing to the position and direction of 

 the opening, and to the Eustachian valve, the arterial 

 blood from the posterior vena cava is directed at once 

 through the foramen ovale into the left auricle, while 

 the venous blood from the right and left anterior venae 

 cavae remains in the right auricle. The right auricle is 

 thus filled with venous blood, and the left auricle with 

 arterial blood. 



" On contraction of the auricles, the blood they contain 

 is driven into the ventricles, so that the right ventricle 

 will be filled with venous, and the left with arterial 

 blood. 



" The left ventricle drives its arterial blood along the 

 systemic trunk, and through the third and fourth pairs 

 of aortic arches to the head and fore-limbs; while the 

 right ventricle forces its venous blood through the pul- 

 monary trunk and fifth pair of arches into the dorsal 

 aorta, from which part goes to supply the body and hind 

 limbs of the embryo, and part, in the earlier stages by 

 far the larger part, passes out along the vitelline and 

 allantoic arteries to the yolk-sac and allantois, where it 

 takes up nutriment and oxygen. 



" The enormously disproportionate size of the head and 

 anterior part of the embryo and the stunted condition 

 of the hinder part during the earlier stages are to be 

 ascribed, at any rate in part, to this arterial supply of 

 the anterior half as contrasted with the venous supply of 

 the posterior half of the embryo." ' 



The changes in actual structure that take 

 place at the time of hatching are comparatively 



' Marshall. 



