160 ORGANISMS ILLUSTRATING BIOLOGICAL PRINCIPLES 



pointed groove which turns the cell to the left as it progresses through 

 the water. When moving into an unfavorable environment or hitting 

 against a solid object, Paramecium reverses the direction of its ciliary 

 lashings, backs away, and goes forward again in a slightly different 

 course, repeating the performance until the obstacle is eventually 

 avoided. Other reactions take place with reference to light, gravity, 

 heat, dissolved chemicals, electricity, and water currents, all of which, 

 whether positive or negative, are co-ordinated by means of a so-called 

 neuromotor mechanism within the cell that enables it to adjust itself 

 to its environment. Under careful methods of staining a number of 

 very minute fibrils may be found in the cell which arise in a central 



I I 



Binary division of Paramecium caiidaiiim. Note the position and structure of 

 micro- and niacronucleus in I. Follow these structures to the formation of the 

 daughter cells (IV). Do both micro- and macronuclei divide by mitosis? What 

 other changes take place in the cells .3 (After Hegner.) 



body near the nucleus and radiate out to the bases of the cilia. This 

 apparatus apparently aids in co-ordinating the action of different 

 parts of the cell. 



Occupying a central area in the cell are two denser bodies, the 

 larger, knowni as the macronucleus, has to do with the metabolic 

 activities of the cell, while the smaller, or micronucleus, contains the 

 chromatic material which is associated with heredity. 



In a hay infusion Paramecia may be found dividing by simple 

 fission. In this process both macro- and micronucleus elongate, and 

 then divide. ' A new gullet buds off from the original one, two new 

 contractile vacuoles appear, and the cell, which has been constricting 

 in the middle, pulls apart to form two new cells. This process may 

 continue for a good many generations where food is plentiful and 

 conditions of life favorable. Woodruff has kept one culture of Par- 



