334 PHYSIOLOGICAL DEVELOPMENT. 



merely a great extension of area in the internal surface of 

 the cul-de-sac, along with that specialization which fits it 

 for excreting and absorbing substances different from those 

 which other parts of the mucous surface excrete and 

 absorb. These lateral air-chambers, universal 



among the higher Vertebrata and very general among the 

 lower, and everywhere attached to the alimentary canal 

 between the mouth and the stomach, have not in all cases the 

 respiratory function. In most fishes that have them they 

 are what we know as swim-bladders. In some fishes the 

 cavities of these swim-bladders are completely shut off from 

 the alimentary canal : nevertheless showing, by the communi- 

 cations which they have with it during the embryonic stages, 

 that they are originally diverticula from it. In other fishes 

 there is a permanent ductus pneumaticus, uniting the cavity 

 of the swim-bladder with that of the gullet: the function, 

 however, being still not respiratory in an appreciable degree, 

 if at all. But in certain still extant representatives of the 

 sauroid fishes, as the Lepidosteus, the air-bladder is " divided 

 into two sacs that possess a cellular structure," and "the 

 trachea which proceeds from it opens high-up in the throat, 

 and is surrounded with a glottis." In the Amphibia the 

 corresponding organs are chambers over the surfaces of which 

 there are saccular depressions, indicating a transition towards 

 the air-cells characterizing lungs; and accompanying this 

 advance we see, as in the common Triton, the habit of coming 

 up to the surface and taking down a fresh supply of air in 

 place of that discharged. 



How are the internal air-chambers, respiratory or noi 

 respiratory, developed? Upwards from the amphibian stage, 

 in which they are partially refilled at long intervals, there is 

 no difficulty in understanding how, by infinitesimal steps, 

 they pass into complex and ever-moving lungs. But 

 how is the differentiation that produces them initiated? 

 How comes a portion of the internal surface to be specialized 

 for converse with a medium to which it is not naturally 



