THE CELL. 29 



The body of the amoeba consists of a gelatinoid substance which 

 has received the name protoplasm, or, more definitely, cytoplasm. 

 Within this cytoplasm and sharply defined from it is a round or 

 oval, vesicular body, called the nucleus, which in turn contains one 

 or more particularly conspicuous granules, the nucleoli. 



The most superficial layer of the cytoplasm appears perfectly 

 clear, colorless, and homogeneous. It envelops the rest of the cyto- 

 plasm, which has a granular appearance. The clear peripheral 

 portion is distinguished as the " hyaloplasm," or " ectoplasm ;" the 

 granular internal portion as " spongioplasm," or " endoplasm." 

 The terms hyaloplasm and spongioplasm are also used in a different 

 and more restricted sense, as will presently appear. 



When viewed under the microscope, the granules of the cyto- 

 plasm are seen to possess a constant, slight, vibratile motion, the 

 Brownian movement, to which is added now and then a flowing 

 movement from one part of the cell to another. At intervals there 

 is a protrusion of the ectoplasm at some point, extending for some 

 distance from the body of the cell, a pseud opodium. This may soon 

 be retracted again, merging with the rest of the ectoplasm, or some 

 of the endoplasm may flow into the central portion of the pseudo- 

 podium, converting it into a broad extension of the cell-body. This 

 may subsequently be withdrawn, or the whole mass of cytoplasm, 

 with the nucleus, may flow into the pseudopodium, gradually in- 

 creasing its size, until the whole cell occupies the original site of the 

 pseudopodium. In this way the animal executes a slow, creeping 

 locomotion. 



These pseudopodial movements and the locomotion occasionally 

 incident to them appear to be wholly spontaneous, i. e. dependent 

 upon internal conditions of which we have no knowledge, but they 

 are manifested only when the amoeba is in contact with some other 

 bodv, not when it is floating freely in a liquid. They may, how- 

 ever, be influenced by external circumstances. Certain substances 

 evidently attract the amoeba, others are either matters of indifference 

 to it or repel it. The chemical nature of the substance or soluble 

 materials emanating from it induce these reactions on the part of the 

 cell. Where this is the case the phenomenon is called chemotropism 

 or chemotaxis, and may be either " positive" or " negative" accord- 

 ing to whether the cell is attracted or repelled. Similar reactions 

 induced by light are included in the term heliotropism, which may 

 be positive or negative. The effect of gravity in its relations to eel- 



