4 THE PROTOZOA THE DAWN OF LIFE 



protoplasmic body mass, but is of a slightly higher refractivity, 

 which renders it conspicuous ; it contains granules of a substance 

 called chromatin, because of the readiness with which it absorbs 

 certain dyes. 



Another minute feature which we shall see in the interior of 

 the Amoeba is a clear, rounded space in the protoplasm, which, 

 as we watch it, will be seen gradually to increase in size up to a 

 certain point when, by a sudden contraction of its walls, it will 

 disappear from view, only to reappear in a short time and again 

 expand. This transparent, pulsating space is called the contractile 

 vacuole, and probably secures to some degree the aeration and 

 purification of the mass of protoplasm. 



If an Amoeba be kept under observation for a little while, the 

 streaming movements of the protoplasm will be seen not only 

 to effect locomotion, but also to bring about the successful cap- 

 ture of food. The drop of water in which the Amoeba is moving 

 contains minute particles of mud, and also probably several of 

 those unicellular plants called " Diatoms," which swarm in both 

 fresh and sea water. Should one of these microscopic plants lie in 

 the path of the Amoeba, the latter will press one of its pseudopodia, 

 or one side of its body, against the Diatom, which will be seen gra- 

 dually to sink into the soft protoplasm and pass into the interior 

 of the Amoeba. The engulfed Diatom becomes surrounded by a 

 little globule of watery fluid, and by degrees the whole of its soluble 

 parts disappear, only the siliceous frustule or skeleton remain- 

 ing. This insoluble skeleton is soon passed outwards from the 

 protoplasm into the surrounding water; and the Amoeba fre- 

 quently appears to get rid of the frustule of the Diatom by the 

 simple process of streaming away from it. 



If food is abundant the Amoeba increases in size, and it is not 

 long before a very remarkable change takes place. The pseudo- 

 podia are withdrawn, partially if not entirely ; the body mass 

 begins to elongate ; and a fissure appears, dividing the Amoeba 

 into two parts. As the waist-like constriction forms, the nucleus 

 may be seen to divide into two, each half moving off into the two 

 lobes of the dividing Amoeba, so that when separation is com- 

 pleted, and we have two distinct Amoeba resulting from the divi- 

 sion of the one, each possesses a nucleus developed from the 

 original nucleus. It is by this simple process of division into 



