BLOOD SYSTEM. 



393 



complex than I have represented, because the amphibian 

 has external as well as internal gills. But the same was 

 doubtless true in the evolution of reptiles. How, then 

 does the change take place ? Fig. 269 represents the cir- 

 culation of the embryo 

 or tadpole, and Fig. 270 

 the adult. In the embryo 

 (Fig. 269), although all 

 the blood goes through 

 the gills, yet note rudi- 

 mentary vessels to the 

 rudimentary lungs, not 

 yet used, and also rudi- 

 mentary vessels connect- 

 ing gill arch to gill arch. 

 Again note in the adult 

 (Fig. 270) remnants of 

 dried-up gill vessels. The 

 letters are the same in 

 the two figures, and the 

 arrows show the direc- 

 tion of the current. 



We have thus proved Fig. 271.— Ideal diagram representing 



the origin of the aortic R^nk?)"^ a ° rtlC afCheS ' (After 



arches of the lizard. 



Now, the same is true of all aortic arches, even those of 

 man himself. 



Birds and Mammals. — In birds and mammals the 

 modification has become so great as to obscure the 

 homology. The proposition to be proved is that the 

 great outgoing vessels of the heart are the modified rem- 

 nants of gill arches. Indeed, this is obvious enough if 

 we take the early embryonic condition of a bird or 

 mammal. The early mammalian, and even human, 

 embryo has gill slits, several of them on each side of 



