192 



THE PROTOZOA 



Plant and animal cells may react differently to the same stimuli. 

 In general, however, we know that protoplasm is irritable to some 

 of these factors. To severe stimuli, protoplasm usually responds 

 by contracting, another power which it possesses. We know, too, 

 that plant and animal cells take in food and change the food to 

 protoplasm, that is, that they assimilate food; and that they 

 may waste away and repair themselves. Finally, we know that 

 new plant and animal cells are reproduced from the original bit of 

 protoplasm, a single cell. 



Reproduction of Paramoecium. Sometimes a paramcecium 

 may be found in the act of dividing by the process known as fission, 

 to form two new cells, each of which contains 

 half of the original cell. This is a method 

 of asexual reproduction, 

 may thus form in succes- 

 sion many hundreds of 

 cells in every respect like 

 the original parent cell. 



Frequently another 

 method of reproduction 

 may be observed. This 



Paramoecium divid- is Called ^jugation, and 



ing by fission, somewhat resembles the 

 Greatly magnified, same process in the simple 



MAC 

 MIC. 



The original cell 



M, mouth; MAC., 



Paramoecium. Greatly 

 magnified. M, 

 mouth ; MIC., micro- 

 nucleus ; MAC., ma- 

 cronucleus ; C. V, con- 

 tractile vacuole. 

 (After Sedgwick and 

 Wilson.) 



macro n ucleus : P lants ' Two cells of equal 



-M7C.,micronucleus. size attach themselves 



(After Sedgwick together as shown in the 

 Figure. Complicated 

 changes take place in the nuclei of the two cells thus united, 

 which results in an equal exchange of part of the material 

 forming the nucleus. After a short period of rest the two cells 

 separate. The stage of conjugation we believed in the plants to 

 be a sexual stage. There seems every reason to believe that it 

 is a like stage in the life history of the paramcecium. 



Amoeba. In order to understand more fully the life of a simple 

 bit of protoplasm, let us take up the study of the amoeba, a type 



1 Amoeba? may be obtained from the hay infusion, from the dead leaves in the bot- 

 tom of small pools, from the same source in fresh-water aquaria, from the roots of 



