264 THE FIFTH DAY. [CHAP. 



whence through the fifth arch it is returned along the 

 aorta to the allantois. 



From the seventeenth to the nineteenth day the 

 right auricle becomes larger than the left. The large 

 Eustachian valve still prevents the blood from the 

 superior cavae from entering the left auricle, while it 

 conducts the blood from the inferior vena cava into that 

 chamber through the foramen ovale. The entrance of 

 the inferior vena cava is however further removed than 

 it was from the foramen ovale, and the increased flow 

 of blood from the lungs prevents all the blood of the 

 inferior cava from entering into the left auricle. At 

 the same time the valve of the foramen ovale prevents 

 the blood in the left auricle from entering the right 

 auricle. 



During the period from the seventh day onwards 

 the apex of the heart becomes more marked, the arte- 

 rial roots are more entirely separated and the various 

 septa completed, so that when the foramen ovale is 

 closed and the blood of the inferior vena cava thereby 

 entirely confined to the right auricle, the heart has 

 practically acquired its adult condition. 



The pericardial and pleural cavities. The heart 

 at first lies in the general body cavity attached to the 

 ventral wall of the gut by a mesocardium (Fig. 86, A), 

 but the part of the body cavity containing it afterwards 

 becomes separated off as a distinct cavity known as the 

 pericardial cavity. It is formed in the following way. 

 When the two ductus Cuvieri leading transversely from 

 the sinus venosus to the cardinal veins become deve- 

 loped (p. 170), a horizontal septum is formed to support 

 them, stretching across from the splanchnic to the so- 



