192 



THE PROTOZOA 



The original cell 



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 paramoecium 

 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 



is called cojijugation, and 



somewhat resembles the 



same process in the simple 



plants. Two cells of equal 



size attach themselves 



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 beheve that it 

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



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 



' Amcebse may be obtained from the hay infusion, from the dead leaves in the bot- 

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



MAC. 

 MIC. 



Paramoecium divid- 

 ing by fission. 

 Greatly magnified. 

 M, mouth; MAC, 

 macronucleus ; 

 M/C,micronucleus. 

 (After Sedgwick 

 and Wilson.) 



Paramoecium. Greatly 

 magnified. M, 

 mouth ; MIC, micro- 

 nucleus ; MAC, ma- 

 cronucleus ; C. V, con- 

 tractile vacuole. 

 (After Sedgwick and 

 Wilson.) 



