THE RHIZOPODA 127 



the biologist, is found in fresh-water ponds and in damp 

 places. There are many kinds of Amoebae, but that kind 

 known as proteus is selected because of its relatively large 

 size and unequivocally simple structure. Amoeba proteus is 

 about '25 mm. (y^th of an inch) in diameter. It is a shape- 

 less mass of protoplasm, which, under moderate magnifying 

 powers of the microscope, appears to be divisible into an outer 

 transparent colourless layer, generally referred to as the ecto- 

 plasm, and an inner, more fluid mass, the endoplasm, contain- 

 ing numerous granules of a dark colour. Closer inspection 

 reveals the presence of a small, subspherical, denser body 

 within the endoplasm ; this is the nucleus, and, if the animal 

 is killed and stained, it is seen, like the nuclei of tissue-cells, 

 to take up the staining fluid with more avidity than the 

 cytoplasm. The nucleus, in fact, contains chromatin, which, 

 in this particular species, is scattered throughout the substance 

 of the nucleus in the form of minute spherules. In addition 

 to the nucleus, clear spaces may be seen in the endoplasm 

 filled with a watery fluid. They are not altogether permanent 

 structures, but are formed or absorbed so slowly that they 

 may be credited with persistent characters, and hence are 

 called permanent vacuoles. Besides these, there is a structure 

 called the contractile vacuole. If a living Amceba is carefully 

 watched under the microscope, a clear round space, larger 

 than the permanent vacuoles, is seen to arise in the cytoplasm. 

 It grows in size, just as a soap-bubble grows when blown from 

 the bowl of a pipe, and its optical characters leave no doubt 

 that it is filled with a watery fluid. After reaching a certain 

 size the vacuole ceases to expand, and, after a period of rest, 

 it is suddenly obliterated by the closing in of its walls from 

 all sides. The obliteration of the vacuole is clearly brought 

 about by the active contraction of the cytoplasm forming its 

 walls, and in the process its fluid contents are expelled, though 

 no obvious pores or canals can be seen through which they 

 effect their escape. 



These are the only definite organs of the Amceba ; but its 

 most characteristic feature is its constantly-changing shape. 

 The creature, under normal circumstances, is never at rest, 

 but is constantly protruding processes of its cytoplasm, now 

 in one direction, now in another. These blunt, shapeless 

 processes are called pseudopodia, and the characteristic move- 



