5© COMPARATIVE ANATOMY 



for a time carried by the female parent to whose back they become 

 attached. The male of the toad, Pipa americana, places the eggs on the 

 back of the female where they sink into the soft skin and become com- 

 pletely enclosed, each egg in an individual pouch. Thus protected they 

 pass through the whole developmental period, omitting the development 

 of the gills which are characteristic of the very great majority of amphibian 

 larvae, and finally rupturing the skin pouches to hop forth as fully formed 

 miniature toads. The female of the South American "marsupial" tree- 

 frog, Gastrotheca (Nototrema), likewise temporarily houses the young 

 on her back, but en masse instead of in individual compartments. A fold 

 of skin opening posteriorly forms a capacious chamber into which the 

 eggs are put. In some species the young emerge from the "marsupial 

 pouch" as tadpoles and in others as fully formed frogs. 



Fig. 35. — Necturus larva of about 25 mm. length. (After Eycleshymer.) 



In a small minority of amphibians, including representatives of each 

 of the three orders, Urodela, Anura and Gymnophiona, the eggs are 

 retained in the oviduct where they are fertilized and pass through the 

 developmental period. This viviparity affords the maximum of protec- 

 tion during development. However, the enclosing of eggs and young in 

 a vocal pouch, in individual dermal sacs, or in a dorsal "marsupium" 

 must be regarded, so far as protection is concerned, as more or less efficient 

 substitutes for true viviparity. 



The amphibian egg with its moderate endowment of yolk, whether 

 laid in the open or enclosed in some protective way, develops rapidly into 

 a highly characteristic larva, the tadpole or "polliwog" (Figs. 31^ and 35) 

 which, with its functional gills and well developed locomotor tail as well as 

 in many features of internal anatomy, is a distinctly fish-like animal and, if 

 its environment is external water, it lives essentially the life of a fish. 

 During the larval or tadpole period, the chief activities are feeding and 

 growth. The end of the larval period is marked by a metamorphosis 

 during which, and in relatively short time, various more or less profound 

 changes occur, thus transforming the animal to the adult type. The trans- 

 formation is most radical in frogs and toads, involving the development of 

 legs and lungs, complete absorption of tail and gills, closure of gill clefts, 

 and other changes. Certain frogs, however, produce especially large eggs 

 which are deposited on land and develop directly into the adult form 

 without passing through a tadpole stage. In the Urodela the changes 



