MECHANICAL MOVEMENTS IN CELLS. 



75 



The contractility of protoplasm may be manifested by either 

 partial or total contractions, the latter tending to cause the protoplasm 

 to assume a spherical shape. Partial contractions are much more 

 common, and consist in contractions along certain circumferences of the 

 mass of protoplasm, and thus lead to the production of irregularity in 

 outline. Movements so produced are described as amoeboid movements 

 from the fact that they are best seen in the amoeba. 



Amoeboid movements have already been described, and are exempli- 

 fied in many of the cells of which the bodies of the higher animals are 

 made up. Thus, the colorless blood-corpuscles, lymph-cells, and corneal 

 corpuscles possess throughout their entire life the power of changing 

 their form in a manner entirely similar to that possessed by the aimsba 

 (Fig. 47). 



FIG. 47. AMCEBOID MOVEMENT IN A COLORLESS BLOOD-CORPUSCLE OF THE 

 FROG. (Engelmann.) 



The temperature was gradually raised from a tow, and then gradually reduced. 



The most striking illustration of this form of protoplasmic move- 

 ment, seen in adult animals, is exemplified in the motions of pigment- 

 cells in the skin of the chameleon. As is well known, the chameleon is 

 capable of changing the hue of its skin, and this is simply due to the 

 varying degrees of contraction of the pigment-cells, which are situated 

 below the epidermis. When these cells send out branching prolonga- 

 tions to the exterior, the skin surface of the chameleon, from the larger 

 amount of pigment exposed, will take on a dark hue. In the different 

 stages of contraction of these pigment-cells the tint of the skin will 

 vary according as the pigment-cells are seen through a thicker or thinner 

 layer of yellowish or almost colorless epidermal cells. 



