Chap. II.] EXTERNAL CHARACTERS. 7 



be compared with the primitive vertebrate position which is 

 well seen in the newt as it lies in a 

 sprawling attitude at the surface of 

 the water (B). In the fore-limb the 

 chief change is the bending forwards 

 of the antibrachium and inwards of the 

 manus, so that the little finger is an- 

 terior. In the hind-limb the femur 

 is swung forwards, and the whole 



.... ., ,- ,., -7 FIG. 3. POSITION OF LIMBS IN 



limb bent upon itself like an Z ; so FROG (A) AND NEWT (B) . 



that the great toe, which in the 



newt is anterior in position, is in the frog, interior. 



The integument of the frog is smooth and moist, and is 

 devoid of scales, feathers, hairs, or any form of exoskeleton. On 

 the dorsal aspect (the back) the colour is yellowish, or reddish 

 brown, with dark brown or greenish spots; on the ventral 

 aspect (the belly) it is pale yellow, with fewer spots. But 

 the colour varies a good deal in different frogs ; and it also 

 varies in the same individual. In bright light the colour be- 

 comes brighter : in the dark it becomes duller. If a frog be 

 kept for some time in a dark cupboard, and then brought out 

 into a bright light, its skin will be found to be dull and pale ; 

 but soon it will become much brighter, and more diversified 

 with spots. 



Examination of a small transparent piece of the skin under 

 the microscope (low power) shows that the colour is due to a 

 great number of minute specks of pigmented material, called 

 pigment cells, or chromatophores (Fig. 24, xii., p. 66). These are 

 of various colours, white, light-yellow, orange, red, brown, and 

 black; but the black will probably be the most conspicuous. 

 Some of them will be rounded or oval ; others star-shaped, or 

 arranged like a piece of network, the fibres of which are of un- 

 equal and inconstant thickness. In some places one colour, and 

 in others another colour, predominates. 



It has been ascertained that the cells change their form in 

 accordance with the brightness of the light that falls upon the 

 eye of the frog. Darkness stimulates the cells to activity, and 



