334 



THE EVOLT^TIOX OF MAN\ 



the iiiuutli ; in this way the circumference of the swini- 

 mincr-blaclcler is diminished, and the fish becomes heavier 

 and sinks. When the animal is again about to ascend, 

 the swimming-bladder is distended by remitting the com- 



'S 



Fig. 284. — Iiitestine of aa embryonic Dog (which is representew in Fig. 

 137, vol. i. p. 382 ; after Bischoff), from the ventral side : a, gill-arches (four 

 pairs); h, rudimentary throat and larynx; c, lungs; d, stomach ; /, liver; t/, 

 walls of the opened yelk. sac, into which the central intestine opens by a 

 wide aperture ; h, rectum. 



Fig. 285. — The same intestine, seen from the right side: a, lungs ; h, 

 stomach ; c, liver ; d, yelk-sac ; e, rectum. 



pressing force. This hydrostatic apparatus begins to be 

 transfoiTTied into a respiratory organ in the Mud-fishes 

 {Dijynettsta), the blood-vessels in the wall of the swim- 

 ming-bladder no longer merely separating air, but also 

 inhaling fresh air, which has come in through the air- 

 passage. This process is fully developed in all Amphibia. 

 The original swimming-bladder here generally becomes a 



