THE EMBRYO. 



293 



ance in absorbing the yelk as nourishment. But in mammals, 

 we have seen that the ovum is minute, and grows by nour- 

 ishment drawn from the mother; and it seems probable that 

 in them the omphalo-mesenteric vessels absorb from other 

 sources besides the contents of the yelk sac. 



214. The heart is at first a straight tube which runs for- 

 wards towards the head, from the point where the omphalo- 

 mesenteric veins unite. But it soon elongates so much that 

 it is thrown into a loop, the prominence of which becomes 

 converted into the ventricular portion of the organ, while 

 the lower part forms the auricular portion, and the upper a 

 common arterial trunk, the truncus arteriosus. Subsequently, 

 the truncus arteriosus is divided longitudinally into aorta 

 and pulmonary artery, and the auricles and ventricles are 



Fig. 157. PLAN OF THE BRANCHIAL ARCHES IN MAMMALS. The 

 portions in outline are obliterated, a, a, a, Aorta ; 6, pulmon- 

 ary artery; c, d, right and left pulmonary arteries; e, truncus 

 arteriosus, obliterated after birth; /, innominate artery; g, g, g, 

 subclaviaii arteries going to the upper limbs ; 7i, h, vertebral 

 arteries ; i, common carotid ; k, k, internal carotid ; m, external 

 carotid artery. Rathke. 



