i86 THE BIOLOGY OF THE FROG chap. 



a burning taste. It may be caused to exude from the skin, 

 especially of the bullfrog, by placing the animal under 

 chloroform. Of its properties in the frog comparatively 

 little is known. Paul Bert found that a goldfinch which was 

 inoculated with the dermal secretion of Rana esculenta died 

 within one minute ; another frog of the same species which 

 was inoculated with the poison died within an hour and a 

 quarter. 



In many other Amphibia, especially the toads and sala- 

 manders, poison glands are very extensively developed, and 

 yield an abundant secretion. 



Sex Differences. — The skin of the frog presents certain 

 differences characteristic of sex, some of which are perma- 

 nent, while others occur only during the breeding period. 

 In Rana fusca^ according to Leydig, and in R. a?'va/is^ 

 according to Steenstrup, the web on the hind feet of the 

 males is more fully developed than in the females. The 

 swelling on the inner side of the first finger of the male, 

 which has been mentioned in a previous chapter, is caused 

 by modifications both of the corium and the epidermis. 

 This swelling is much larger in the breeding period than at 

 other times, and it doubtless subserves the function of aid- 

 ing the male to retain hold of the female. The cutaneous 

 glands in this region are much enlarged, and become elon- 

 gated into a tubular form, and extend through the entire 

 thickness of the skin. The epidermis in the breeding period 

 is proliferated to form small papillae with a thick, rough, 

 horny layer. After the breeding period the epidermis 

 becomes smooth again, and there is also a partial disappear- 

 ance of the pigment of the corium, so that the swelling loses 

 its dark color. 



The occurrence of dermal papillae in the female of Rana 

 fusca during the breeding period has already been sufficiently 



