476 ZOOLOG r. 



acquire the dorsal crest and a broader tail-fin, while in some 

 species prehensile claws are temporarily developed on the fore 

 legs of the male. The males of the Anura (toads and frogs) 

 are musical, the females being comparatively silent ; the vocal 

 organs of the male are more developed than in the females, and 

 in the edible frog (Rana esculenta) large sacs for producing 

 a greater volume of sound stand out on each side of the head 

 of the males. Among the few viviparous Batrachians known 

 is an Alpine European Salamandra (S. atrd) which brings 

 forth its young alive. 



It is common to find tadpoles in the winter in ponds, 

 which have been retarded in their metamorphosis, and by- 

 artificial means this retardation may be greatly increased. 

 For example, Wyman is said to have kept tadpoles of the 

 bull-frog for seven years in a cellar. 



O / 



Unlike the higher Vertebrates the segmentation of the egg 

 in the Amphibia is total, the process beginning usually about 

 three hours after impregnation in the frog, and lasting twen- 

 ty-four hours. The primitive streak, the notochord and 

 nervous system then arise as in other craniated Vertebrates. 

 After the appearance of the branchial arches, the gills begin 

 to bud out from them, finally forming the larger gills of the 

 tadpole. Unlike young fishes, the yolk is entirely absorbed 

 before the tadpole leaves the egg. In warm climates the 

 tadpoles hatch in four or five days after the eggs arc laid. 

 When hatched the tadpole is not so well developed as in most 

 young fishes. The digestive canal at first is simple and 

 straight. Afterwards it becomes remarkably long and coiled 

 in a close spiral. The mouth is small (Fig. 434, A), with 210 

 tongue and only horny toothless jaws. The vertebrae of the 

 tadpole are biconcave as in fishes, afterwards becoming con- 

 verted into cup-and-ball joints. 



The accompanying figures represent the external changes 

 of the toad from the time it is hatched until the form of the 

 adult is attained. The tadpoles of our American toad are 

 smaller and blacker in all stages of growth than those of the 

 frog. The tadpole is at first without any limbs (Fig. 434 A), 

 and with two pairs of gills ; soon the hinder legs bud out. 

 After this stage (B) is reached, the body begins to diminish in ' 



