EVOLUTION OF THE LUNGS. 335 



lung, and its air-passage a wind-pipe. The amphibian lung 

 has been transmitted to the three higher vertebrate classes, 

 and even in the lowest Amphibia the lung on either side 

 is as yet a very simple, transparent, thin-walled sac — as, 

 for instance, in our common Water-Newts, or Tritons, and 

 very like the swimming-bladder of Fishes. The Amphibia 

 liave, it is true, two lungs, a right and a left; but in 

 many Fishes also (in the ancient Ganoids) the swim- 

 ming-bladder is double, the organ being divided into a 

 right and a left half. On the other hand, the lung of the 

 Ceratodus is single (p. 119). The earliest rudiment of the 

 lung in the human embr3^o and in the embryo of all higher 

 Vertebrates is also a simple, single vesicle, which does not 

 separate till afterwards into a pair of halves — the right and 

 the left lung. At a later period, the two vesicles grow con- 

 siderably, occupy the greater part of the chest cavity, and lie 

 one on each side of the heart ; even in Frogs we tind that the 

 simple sac, in the course of its development, is transformed 

 into a spongy body of a peculiar, froth-like texture. This 

 lung-tissue develops as a tree-like, branched gland, bearing 

 berry-like appendages. The process by which the lung-sac 

 was attached to the anterior intestinie, which was originally 

 very short, lengthens, by simple growth, into a long thin 

 tube ; this tube is the wind-pipe (trachea) ; it opens above 

 into the throat (jjharynx), and below divides into two 

 branches which pass into the two lungs. In the wall of the 

 wind-pipe ring-shaped cartilages develop, which keep the 

 wdiole distended ; at the upper end of this wind-pi})c, below 

 its entrance into the throat, the larynx, the organ of voice 

 and speech, develops. The larynx occurs even in Amphibia 



in very various stages of development, and with the aid of 

 55 



