STRUCTURE OF LIVING THINGS 179 



minute drop of human blood can be kept alive for hours, 

 if the glass slide is kept warm, as it easily can be, and 

 I have seen the cells in a drop of frog's blood (skilfully 

 treated) still alive, and exhibiting active movements, a 

 fortnight after the frog, from which the drop of blood 

 came, was dead and buried. These floating, moving 

 cells of the blood are the " phagocytes," which engulf 

 and digest disease germs and other particles (Fig. 36 B). 

 Other more numerous cells of the blood are the oxygen- 

 carriers, or red corpuscles, which do not show any move- 

 ments or changes of an active kind whilst alive. 



