39 2 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



state it is essentially a fish. Now this fishlike gill-breath- 

 ing animal changes into a lung-breathing land animal, 



Fig. 269. 



Fig. 270. 

 Figs 269, 270. -Diagrams showing the change of the course of blood in the 

 development of a frog Fig. 269 the tadpole stage. Fig. 270, the ma- 

 ture condition: H, heart; GG'G", external gills; ^V',' internal 

 gills; cc, connecting branches in the tadpole ; pp, pulmonary branchef. 



and in doing so the gill arches are changed into aortic 

 arches before our eyes. The process is a little more 



