SECTION II 

 THE CELL 



PROTOPLASM occurs in the animal body as small separate masses 

 or CELLS. These vary considerably in size, but, on an average, 

 they are from 7 to 20 micro-millimetres l in diameter. The 

 advantage of this subdivision is obvious. It allows nutrient 

 matter to reach every particle of the protoplasm. In all higher 

 animals each CELL has a perfectly definite structure. It con- 

 sists of a mass of protoplasm, in which is situated a more or less 

 defined body, the nucleus. 



(A) Cell Protoplasm 



This has the structure already described under protoplasm, 

 and in different cells the reticulum or cytomitoma is differently 

 arranged. In some cells there is a condensation of the re- 

 ticulum, round the periphery, to form a sort of cell membrane. 



At some point in the protoplasm of many cells, one or two 

 small spherical bodies, the eentrosomes (fig. 2), are found, from 

 which rays pass out in different directions. For the detection 

 of these bodies special methods of staining and the use of very 

 high magnifying powers are required. They will be again 

 considered when dealing with the reproduction of cells. 



The cell protoplasm frequently contains granules, either 

 formed in the protoplasm (p. 23), or consisting of material 

 ingested by the cells. 



In the protoplasm, vaeuoles are sometimes found, and from 

 a study of these vaeuoles in protozoa, it appears that they are 

 often formed round material which has been taken into the 

 protoplasm, and that they are filled with a fluid which can 

 digest the nutritious part of the ingested particles. In some 

 cells, vaeuoles may appear in the process of disintegration. 



In certain cells protoplasm undergoes changes in shape. 

 This may well be studied in the white cells in the blood of 



1 The micro- millimetre is the mVuth of a millimetre. 



