240 Basic Structure of Vertebrates 



Fig. 203. Necturus larva of about 25 mm. length. (After Eycleshymer. Courtesy, 

 Neal and Rand: "Chordate Anatomy," Philadelphia, The Blakiston Company.) 



lives the life of a fish. The larval period, ranging from a few weeks in 

 some salamanders to a year or more in some frogs, is devoted mainly 

 to feeding and growth. It terminates in a metamorphosis, in the course 

 of which the animal acquires the adult characteristics. The transforma- 

 tion is most radical in frogs and toads; legs and lungs develop, tail 

 and gills are absorbed, gill-clefts close, and other changes occur. In 

 certain exceptional species of frog, especially large eggs are laid on 

 land and develop to adult form without passing through a tadpole 

 stage. In the Urodela the changes are less marked, the tail and some- 

 times also the gills being retained. Adult Necturus, with its tail and 

 functional gills, is sometimes called a "permanent larva." Sexual 

 maturity is ordinarily not attained during the larval state. But the 

 Mexican axolotyl, the larva of the salamander Ambysloma tigrinum, 

 regularly breeds in the larval state. 



The diversity of reproductive arrangements in amphibians is in 

 marked contrast to the uniformity which prevails in reptiles and 

 birds. All oviparous reptiles, even those that are aquatic (alligator, 

 turtle), and all birds, deposit the eggs on land. The large yolk-mass 

 of the eggs of these animals makes possible a long developmental period 

 during which the young can attain relatively great size. A newly 

 hatched alligator is gigantic compared to a newly hatched salmon. 

 These circumstances, together with the fact that development takes 

 place within a thick shell, make necessary some special provision 

 whereby food derived from the yolk may be made accessible to all parts 

 of the large embryo and an adequate supply of oxygen obtained from 

 some external source — an especially urgent need in warm-blooded birds. 



The outstanding feature of the development of the reptile or bird 

 appears when the embryo itself goes about the business of constructing 

 a complex system of membranes so disposed and so equipped with 

 blood-vessels as to serve very efficiently not only for respiration but 

 also for some other and secondary functions. 



Early in development, at a time when the main organs are in process 

 of formation (Fig. 239), the outer layer of the embryo, representing the 

 prospective body-wall of the animal, throws up a system of folds which 

 arch over and ultimately enclose the whole of the definitive embryo — 



