20 BIOLOGY 



more dense arrangement in the protoplasm, we pass to 

 forms where there is a very delicate cuticle or pellicle, 

 as in paramoecium, and finally to such cells as the 

 encysted stages of many of the lower forms, and the 

 spores of bacteria where the cyst or cell wall has become 

 a dense, highly resistant structure. The highest develop- 

 ment of cell wall is seen in the higher plants, where it 

 may be said to be a product of the cell rather than a 

 part of the cell. In the growth of a higher plant the 

 cell walls, which were thin at the beginning, become 

 greatly thickened by the deposition of new layers, and 

 this thickening may result in the formation of cellulose, 

 and finally in such substances as lignin and suberin. 



One other structure that occurs in many cells is a 

 small body called a centrosome ; in some cells it appears 

 just before division, in others it is a permanency. Its 

 exact functions are unknown, but it apparently has 

 something to do with the process of division, for it 

 divides always before any of the other structures divide. 



There are, in many cells, structures called vacuoles. 

 In vegetable cells they are probably drops of sap which 

 distend the cell during nutrition. In animal cells the 

 vacuoles seem to be different, and are often formed in 

 definite places. There they may help in the circulation 

 of the digested food material and so assist assimilation, 

 or they may act to a certain extent as organs for the 

 excretion of waste nitrogenous matter. 



These cell-vacuoles are in many cases simply stores 

 of reserve material, and this is well illustrated by the 

 globules of fat and glycogen found in the liver cells of 

 man. 



