46 ORGANISMS OF ONE CELL 



while the rapidly moving forms are relatively clear and trans- 

 parent (Fig. 1 8). 



Nucleus. While the form of Amoeba proteus is continually 

 changing, the structure remains practically the same. It is 

 always one protoplasmic unit, a cell, and like other cells it is 

 differentiated into cell body, called cytoplasm, and nucleus. 

 This nucleus is composed of slightly different protoplasm from 

 that of the cell body, and is chiefly characterized by the greater 

 affinity for basic stains, while the protoplasm of the cell body 

 has an equivalent affinity for acid stains. The substance of 

 the nucleus which takes the stain is called chromatin, and is 

 composed chiefly of nucleins or nucleo-proteins particularly 

 rich in phosphorus. The nucleus can be seen in life as a pale 

 circular disc moving with the flow of granules from one part of 

 the cell to another, and turning as it moves, showing now the 

 circular outline, again the flattened edge view. 



Endoplasm and Ectoplasm. The cytoplasm is not entirely 

 homogeneous but is 'differentiated into an inner and an outer 

 portion. The former, in which the nucleus is found is called 

 the endoplasm, or sometimes, the endosarc. The latter, called 

 the ectoplasm or ectosarc, although soft and gelatinous, is firmer 

 and denser than the endoplasm and is more transparent, for 

 it has none of the refractive bodies found in the endoplasm. It 

 is to be regarded as a protective layer, since it is the part that 

 comes in contact with the surrounding medium, and through 

 it all intercourse between the amoeba and the environment 

 must take place. In other forms of protozoa it is this ecto- 

 plasm which becomes differentiated in the greatest variety of 

 ways for purposes of protection, locomotion, and sensation. 



The endoplasm, on the other hand, contains the vital organs 

 of the cell, a number of which can be seen with little effort. 

 Large particles more or less disintegrated, and surrounded by 

 clear fluid, are food bodies recently ingested and are undergoing 

 digestion in the fluid-filled spaces, which, for this reason, are 

 called gastric vacuoles. The bodies in these vacuoles may fre- 

 quently be recognized as portions of other minute animals or 

 plants. Another fluid-filled sphere, usually best seen near the 



