XII.] ARTERIAL SYSTEM. 407 



and downwards, and eventually meet on the ventral 

 side of the throat. For a short time they here remain 

 distinct, but soon coalesce into a single tube. 



In Birds, it will be remembered, the heart at first has the 

 form of two tubes, which however are in contact in front. It 

 arises at a time when the formation of the throat is very much 

 more advanced than in Mammalia ; when in fact the ventral 

 wall of the throat is established as far back as the front end of 

 the heart. 



In the lower types the heart does not appear till the ventral 

 wall of the throat is completely established, and it has from the 

 first the form of a single tube. 



It is therefore probable that the formation of the heart as two 

 cavities is a secondary mode of development, which has been 

 brought about by variations in the period of the closing in of the 

 wall of the throat. 



The later development of the heart is in the main similar to 

 that of the chick (p. 256 et seq.). 



The arterial system. The early stages of the 

 arterial system of Mammalia are similar to those in 

 Birds. Five arterial arches are formed, the three poste- 

 rior of which wholly or in part persist in the adult. 



The bulbus arteriosus is divided into two (fig. 137 

 B), but the left fourth arch (e), instead of, as in Birds, 

 the right, is that continuous with the dorsal aorta, and 

 the right fourth arch (i) is only continued into the right 

 vertebral and right subclavian arteries. 



The fifth pair of arches which is continuous with 

 one of the divisions of the bulbus arteriosus gives origin 

 to the two pulmonary arteries. Both these however are 

 derived from the arch on one side, viz. the left (fig. 137 

 B); whereas in Birds, one pulmonary artery comes from 

 the left and the other from the right fifth arch (fig. 

 137 A). 



