MAMMALIAN GILL ARCHES 



401 



ancestors. The blood passes through these vessels for a time, 

 but soon an entire rearrangement takes place, and in Birds and 

 Mammals the connection between the heart and dorsal aorta 



Fig. 249. — Diagram of the human embryo to show the arrangement of the 

 blood-vessels. E, Eye; 0, ear; Mn, lower jaw; H, heart. From the heart a 

 large vessel leads forward and then divides into five pairs of vessels (only the 

 five of one side are represented). These vessels pass over the gill arches to the 

 dorsal side and there unite to form the dorsal aorta. In the adult, however, 

 only the fourth branch of the left side remains to connect the heart with the 

 dorsal aorta. (From McMurrich after His.) 



is maintained only by a single vessel. In Birds this vessel 



Hes on the right side, in Mammals on the left. The other vessels 



either become connected with other parts of the body or else 

 26 



