212 VERTEBRATE ZOOLOGY 



a vibratile tail, and the capture of food both by sharply pointed, re- 

 curved teeth and by the claws of a five-fingered hand and foot." 



The essential evolutionary advance which the reptiles made upon 

 the Amphibia had to do with the total abandonment, from the very 

 beginning of development, of the aquatic habit of hfe. The Am- 

 phibia were, and still are, for the most part, dependent upon water dur- 

 ing at least a considerable portion of their Hfe cycle; the ReptiUa were 

 from the first quite independent of an aquatic environment. This 

 emancipation from the need of an aquatic habitat was accomphshed in 

 three ways: by the acquisition of limgs of a more adequate sort; by a 

 scaly covering to prevent dessication; by the development of impor- 

 tant embryonic membranes, the amnion and the allantois, which 

 are merely adaptations for embryonic development in the air. 



Undoubtedly the Amphibia had made considerable progress in the 

 direction of adaptations for adult life on land, for they often have as 

 good lungs as do some of the reptiles. Moreover, many of the ex- 

 tinct Amphibia had an adequate scaly covering. There is no evi- 

 dence, however, that any of the Amphibia have ever acquired the 

 capacity of reproducing on the dry land, except in connection with 

 some peculiar brooding adaptation, such as that seen in some of the 

 tree-toads. The fact that all modern Amphibia have functional gills 

 at some period adds force to this last statement. 



The reptiles have entirely given up gill respiration, as is evidenced 

 by the total absence, except for minute transitory traces, of gill 

 tissue at any stage of development. In place of gills the embryo uses 

 the allantois, an extensive embryonic lung. To avoid dessication and 

 the possibility of contact injuries, the embryo is surroimded by a 

 fluid-filled sac, made from folds of its own tissues. This veritable 

 private aquarium is called the amnion, a structure adopted by all 

 land vertebrates for development in the air. 



The amnion, and the allantois then are structures of first rate 

 importance in connection with the origin and evolution of land verte- 

 brates, and especially of the Reptilia. The egg of the reptile is a 

 large object, much like that of the bird. It is provided with a tough 

 shell for protection and a thick layer of albumen for nutriment. The 

 egg-yolk is abundant and serves as the main food supply of the grow- 

 ing embryo. There is no real larval stage, for the newly hatched 

 young is essentially like the adult except in the relative proportions 

 of head and body and in being sexually immature. ' 



