THE CELL 101 



cell, in the sense of protoplasmic mass, thus comes to 

 lie loosely in a circumscribed space, the walls of which 

 are of its own formation, but to which it is not indis- 

 solubly bound and of which it is not itself a part. 



Thus it appears that the botanist and the zoologist 

 mean somewhat different things when they speak of the 

 cell wall. ' 



Many cells contain a small rounded body known as 

 the centrosome. It is subject to many variations. Some- 

 times it appears shortly before cell division is to take 

 place; sometimes its presence is constant. 



The source and function of this body are unknown. 

 It is in some manner connected with multiplication, for 

 it always divides before the other structures of the cell, 

 and in many cases it disappears shortly after the process 

 of cell division has been completed. 



Vacuoles are frequently present in the cells. The 

 term is applied to what seem to be empty spaces 

 in the cytoplasm. They are most frequent and largest 

 in vegetable cells during active growth and then are 

 really drops of sap with which the cytoplasm distends 

 itself during the process of nutrition. 



In animal cells similar vacuoles are to be found and 

 are probably formed by the local collection of the prod- 

 ucts of digestion awaiting assimilation. In certain 

 of the protozoa these products appear to collect in 

 preformed spaces which communicate with one another 

 so that the contents of one may be expelled into another. 

 In some of these animals there is a rhythmical to-and- 

 from movement of the fluid from one vacuole to another, 

 contractile vacuoles. It is supposed that the movement 

 of the fluid aids in assimilation through the acceleration 

 of cytoplasmic circulation. 



Vacuoles not infrequently consist of reserve stuffs 

 temporarily stored in the cells. Of such nature are the 

 globules of fat and glycogen so commonly found in the 

 liver cells of man, and the enormous fatty globules in the 

 fat-storing organs and subcutaneous tissue of the higher 

 animals. 



