EVOLUTION OF AMNION ANBIALS. I35 



Animals. All these, even such as live in the water, e.g., 

 whales, respire only air through lungs, never water through 

 gills. While all Amphibians, with very few exceptions, in 

 tlie young state retain their gills for a longer or shorter 

 period, and breathe through gills for some time (if not 

 always), from this point gill-respiration entirely ceases. 

 Even the Protamnion must have entirely ceased to breathe 

 water. The gill-arches, however, remain, and develop 

 into very different organs (partly rudimentary) ; into the 

 various parts of the tongue-bone, into certain portions of the 

 jaw apparatus, the organ of hearing, etc. But no trace of 

 gill-leaves, of real respiratory organs on the gill-arches, are 

 ever found in the embryo of Amnion Animals. 



With this total loss of the gills is probably connected 



the formation of another organ, which we have already 



described as occurring in human Ontogeny ; this is the 



allantois, or primitive urinary sac. (See vol. i. p. 379.) In 



all probability the urinary bladder of the Dipneusta is to be 



regarded as the first beginning of the allantois. Even in 



the American Mud-fish {Lepidosiven) we find an urinary 



bladder, which grows from the lower wall of the posterior 



extremity of the intestine, and serves as a receptacle for 



the renal secretions. This organ has been inherited by the 



Amphibia, as may be seen in any Frog. But it is only in 



the three higher Vertebrate classes that the allantois attains 



a special development ; in these it protrudes at an ear];y 



period from the body of the embrj^o, forming a large sac 



filled with liquid, and traversed by a considerable number 



of large blood-vessels. This sac also discharges a portion of 



the nutritive functions. In the higher Mammals and in 



Man the allantois afterwards forms the placenta. 



