THE WHITE BLOOD CELLS. 



229 



rapidly past them down the centre of the channel (Fig. 98). 

 This may partly be due to their peculiar adhesiveness, which 

 also causes them to stick to the glass slide, while the red disks 

 are washed away when a little stream of saline solution is allowed 

 to flow under the cover glass. These cells show all the manifes- 

 tations of activity characteristic of independent living beings. If 

 kept in a medium suitable to them, and at the temperature of 

 the body, they will be seen soon to alter their appearance ; their 

 outline becomes faint, they are no longer spherical, but very 



FIG. 98. 



Vessels of the Frog's Web. (a) Trunk of vein, and (b b) its tributaries passing across 

 the capillary network. The dark spots are pigment cells. 



irregular in shape, and constantly change their form by sending 

 out and retracting delicate processes, by means of which they 

 change their position, so that they may be said to perform loco- 

 motion. These movements are rendered more active by a slight 

 increase of temperature, and are checked by cold. For continued 

 observation, about 38 C. is the best temperature. They respond 

 to many other influences, such as electricity, etc., even for a con- 

 siderable time after removal from the body. 



