THE BUILDING-STONES OF THE ORGANIC WORLD 



73 



Like persons promenading, these granules slowly wander away 

 from the body on one side of the little foot, and then turn round 

 and come back on the other side. But as all persons do not walk 

 at the same pace, so of these granules some travel quickly, some 

 slowly ; sometimes one overtakes the others, then apparently rests 

 itself and yields the lead to others. We are able to influence the 

 rapidity of motion, increase it or arrest it, through stimuli of 

 different kinds, chemical influences, or the effects of warmth 

 or cold. This is the best proof that protoplasm is irritable, 

 that it possesses sensation. How, indeed, could life exist without 

 the possibility of purposeful reaction, and how, again, could this 

 be possible without sensation. If the Amoeba felt no sensation of 

 hunger, what could suggest to her the "thought " of eating ? 



These animalcules have no oral aper- 

 ture, but they are able to absorb nourish- 

 ment in an exceedingly simple manner. 

 When on its chase the Amoeba encoun- 

 ters an organism suitable for food, it seizes 

 it with its pseudopodia and wraps itself 

 around it. Then in the interior of its 

 protoplasm all the nutritive parts of the 

 prey are extracted and the indigestible re- 

 mainder ejected. Certain cells of the 

 body of higher animals the white blood- 

 corpuscles or leucocytes feed in this 

 primitive manner, to the great advantage 

 of the organism, for the leucocytes repre- 

 sent, as it were, the natural bodyguard 

 whose duty is to take care that no dan- 

 gerous germs settle down in the body. Differing but slightly 

 in outward appearance from the Amoebae, and being, like them, 

 motile, the leucocytes leave the blood-channel and traverse all 

 the tissues of the body. When on these scouting expeditions 

 they encounter any bacteria or any other minute parasites they 

 dispose of them by the simple process of eating them up. Many 

 illnesses are probably warded off by the watchfulness of these 

 phagocytes (see fig. 21). The leucocytes are also believed to 

 fulfil a useful part in the nutrition of the body-cells by charging 

 themselves with liquid food from the intestinal glands and 



FIG. 21. LEUCOCYTES OF 

 THE FROG DEVOURING A 



(After E. Metchnikoff.) 



