Ill EXTERNAL CHARACTERS OF THE FROG 67 



the exception of small scattered prominences occurring 

 mainly on the back and on the dorsal side of the hind legs. 

 Nothing corresponding to hair or scales is to be found in 

 the frog; in fact, with the rare exception of rudimentary 

 scales in some forms, such structures are entirely absent from 

 all of the recent Amphibia. The general looseness of the 

 attachment of the skin is a feature which cannot fail to be 

 noticed. 



Behind the eyes there extend two, usually light-colored, 

 ridges formed by a thickening of the skin, and known as 

 the dorso-laieral dermal plicce or folds. There are usually 

 several smaller and somewhat irregular longitudinal folds of 

 skin between these. The color of the skin is much darker 

 on the upper or dorsal side than below, where it is entirely 

 white. The large black pigment spots which occur on the 

 dorsal side of the body and legs are subject to much varia- 

 tion in size and shape. There is usually a pair of large 

 spots between the eyes and a single median spot in front of 

 these. The spots between the dorso-Iateral folds show a 

 tendency to arrange themselves in two rows. The spots on 

 the hind legs are frequently elongated so as to form trans- 

 verse bands. In addition to the black pigment there are 

 green and golden colors, which are present in varying 

 proportions. The changes in color which the skin may 

 undergo under certain conditions will be discussed in a later 

 chapter. 



