chap, v.] FIBROUS CONNECTIVE TISSUES. 39 



exceedingly numerous, and form a network so dense 

 that the whole mass of cells resembles an extremely 

 close network of pigment. In the maximum of 

 activity the processes disappear, being withdrawn into 

 the cell-body, which now looks like a spherical or 

 oblong mass of black pigment. Between the states 

 of passiveness and maximum activity there are various 

 intermediate grades, in which the pigmented processes 

 are of various numbers and lengths. 



44. Owing to the great number of the pigment 

 cells in the skin of fishes and amphibian animals, the 

 state of contraction of these cells materially affects 

 the colour of the skin. If the dark pigment cells 

 of a particular part contract, the skin of this parti- 

 cular part will become lighter and brighter, the degree 

 of lightness and brightness depending on the degree 

 of contraction of the pigmented processes by the cells. 

 Briicke has shown that darkness is a stimulus to the 

 pigmented cells ; they contract, and the skin becomes 

 light. Sunlight leaves the pigmented cells in the 

 passive state, i.e., the skin becomes dark. If pre- 

 viously they have been contracted by darkness, on 

 being exposed to sunlight they again return to the 

 passive state. The contraction of the pigment cells 

 is under the direct influence of the nervous system 

 (Lister). Pouchet proved that the contractility of the 

 pigment cells of the skin of certain fishes is influenced 

 as a reflex action by the stimulation of the retina by 

 light. 



45. Fat Cells Fat cells in the ripe and fully- 

 formed state are spherical, relatively large vesicles, 

 each consisting (a) of a thin protoplasmic membrane, 

 which at one point includes an oval nucleus flattened 

 from side to side, and (b) of a substance, which is an 

 oil globule filling the cavity of the vesicle (Fig. 28). 

 These fat cells are massed together by fibrous connective 

 tissue into smaller or larger groups, which in their 



