THE CELL. 29 



moist earth, upon the surfaces of aquatic plants, and in the soil at 

 the margins of ponds and sluggish streams. 



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 pseudopodium. 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 may subsequently be withdrawn, or the whole mass of cytoplasm, 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. They 

 may, however, be influenced by external circumstances. Certain sub- 

 stances evidently attract the amoeba, others are either matters of in- 

 difference to it or repel it. If a pseudopodium comes in contact with 

 some particle in the surrounding medium, it may retreat from it, 

 appear indifferent to it, or be attracted and proceed to incorporate 

 it. This is accomplished by the cytoplasm flowing around the for- 

 eign body and coalescing on its further side so as to enclose it. It 

 is then conveyed to the body of the cell, either by cytoplasmic cur- 

 rents, by the withdrawal of the pseudopodium containing it, or 



