76 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



The two extremes of position of these pigment-cells are represented 

 in Figs. 48 and 49. 



FIG. 48. 



FIG. 49. 



PIGMENT LAYER OF THE SKIN OF THE CHAMELEON IN DIFFERENT DEGREES 

 OF CONTRACTION. (Briicke.) 



The frog also, as is well 

 known, is another illustration 

 of a change of tint produced 

 by precisely similar processes. 



From the fact that the 

 movements of these pigment- 

 cells appear to be under the 

 control of the nervous system, 

 they offer an illustration of a 

 transition stage between the 

 independent, automatic move- 

 ment of free protoplasm, as in 

 the body of the amoeba, and 

 the specialization of the func- 

 tion of movement in the nerv- 

 ous ganglia, nerves, and muscle- 

 cells of higher animals. 



In certain of the low forms 

 of life protoplasmic motion 

 may take on the form of minute 

 contractile threads thrown out 

 from the body of variovis rhiz- 

 opods and monera, which dif- 

 ers from the amoeboid move- 

 ments just described. In this 

 case long and thin protoplasmic 

 threads in great number extend in every direction from the central mass ; 

 these threads, on whose surfaces fine granules are often seen in active 



FIG. 50. 



