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48 PROTOPLASM AND THE CELL 



The ameba is generally taken as the most typical and the most simple 

 of animal cells. It has for us additional value for the reason that it is 

 endowed with more functions than many sorts of tissue-cells, because 

 it is an independent and separate animal. It is not a part of a tissue 

 with only one or two processes to accomplish e. g., an epithelial cell in 

 a mammal. There are several species of the genus ameba, but the one 

 most typical, largest, and best adapted for description is ameba proteus,. 

 classed zoologically as a protozoan rhizopod. These animals are often 

 large enough to be seen by the unaided eye, but more usually are only a 

 small fraction of a millimeter in diameter, and therefore require much 

 magnification for study. When enlarged about three hundred diameters 

 there can be seen a mass, irregular in shape, of a substance which looks 

 like granular jelly in the bottom of the water drop on the slide of the 

 microscope. Around the edges of the mass the jelly-like protoplasm is 

 freer of granules than it is within. This more or less transparent tem- 

 porary edge of the animal is called the ectoplasm, and the inner and more 

 granular portion the endoplasm, distinctions which are of slight signifi- 

 cance. Near the centre of the animal is a small spherical body, often 

 darker than the surrounding protoplasm, which may be encircled by a 

 more or less transparent ring. This rounded mass is the nucleoplasm 

 or nucleus. The rest of the cell is called the cytoplasm. Within the 

 nucleus there is, although exceeding small, a still darker dot, which is 

 the nucleolus, of unknown significance. About the nucleus, or here and 

 there through the cytoplasm generally, are seen, sometimes prominently, 

 what appear like bubbles, but which are spaces filled with clear liquid, 

 but not a gas. Most of these vacuoles, as they are mistakenly called , 

 are constant and unchangeable in size, and are termed permanent 

 vacuoles. One, larger than the rest ordinarily, may be seen to grow 

 slowly and to disappear suddenly (by bursting) when it has reached a 

 certain size; this is the contractile vacuole. Close examination shows 

 the whole nucleus to be pervaded with small masses of a more opaque 

 substance called chromatin, while with a very high magnification, the 

 cytoplasm, apparently homogeneous with a low power, shows a minute 

 reticular structure, as if made up of a mass of liquid foam. The status 

 of the distinct granules is not yet clear. Despite their name, metaplasm, 

 they probably are a part of the protoplasm. These, then, are the 

 "organs" of ameba in its normal condition. Sometimes other objects 

 within its mass may be noted. For example, there may be seen par- 

 ticles of food (a vegetable cell or a diatom) or a piece of the waste left 

 from the digestion of such a meal. These particles, while in process of 

 digestion, may be surrounded by visible vacuoles, in which case the 

 latter are filled with digestive juices. The finer structure and signifi- 

 cance of these various organs will be described later, our endeavor now 

 being to gain an understanding of how protoplasm appears and what 

 animals made of it do. (See Fig. 1.) 



It would be difficult to distinguish an ameba from the multifarious 

 debris of the pool-bottom in which it is to be found were it not that obser- 



