THE PROTOZOA 



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very tiny, one of large size reaching only about 0*25 mm., and so 

 it needs a lens to see it at all. 



Under the microscope Amoeba appears as a mass of fairly trans- 

 parent granular jelly continually changing in shape by the pro- 

 trusion of blunt processes, the pseudopodia, as they are called. 

 In many ways it recalls the leucocyte of the frog's blood, but it is 

 much larger and more active. Its protoplasm, which is not pro- 

 tected by any form of skeletal structure, is clearly marked off into an 

 outer and an inner portion. The outer layer, the ectoplasm, is thin 

 and transparent, and although soft, is firmer than the underlying 

 substance, so forming a protective covering. Inside this, and form- 

 ing the bulk of the cell, is the endoplasm, of a more fluid consistency 



FIG. 37. Different forms assumed by Amoeba proteus. Photographs 

 from preparations. From Calkin. 



and less transparent, owing to the presence in it of a number of 

 refractive granules. All the vital organs of the cell lie in it, and it 

 flows about freely within the ectoplasm as if confined in a sort of 

 bag. Somewhere near the middle of the cell is the nucleus, a 

 spherical structure not readily seen until the animal is killed and 

 stained. The nucleus is clearly delimited from the remaining 

 endoplasm by a very thin homogeneous membrane, the nuclear 

 membrane. The protoplasm within the membrane is practically 

 indistinguishable from that outside, but contains a large number of 

 minute granules of an important substance termed chromatin, 

 because of the readiness with which it takes up certain basic stains. 

 This chromatin, which is in Amoeba fairly evenly distributed, thus 



