218 



Marsh. 



upper chamber of the heart becomes divided into two by the 

 growth of a median partition, whilst in the upper or thoracic 

 part of the body, two bag-like organs, composed of exceedingly 

 fine membrane, grow. These become the lungs, and the blood 

 which has before been pumped to the gills by the heart is 

 now driven into the delicate membrane of the lungs instead, 



if* 1 1 



1 / 9 * 



FIG. 48. EARLY STAGES OF TOAD. 



a, b, Eggs of toad, c, d, Embryo in egg before hatching. 



e, f, g, fc, Newly -hatched tadpoles, i, j, fc, Tadpoles in various stages of 



development. 



where it is purified by absorbing some of the oxygen 

 from the air which the hollow sacs of the lungs contain, and 

 by passing the effete and used-up gases which it carries 

 through the thin membrane of the lungs in the opposite 

 direction, whence they are expelled from the body. 



