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 in 

 diameter. The advantage of this subdivision is obvious. 

 It allows nutrient matter to reach every particle of the proto- 

 plasm. In all higher animals each CELL has a perfectly 

 definite structure. It consists 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 reti- 

 culum or cytomitoma is differently arranged. In some cells 

 there is a condensation of the reticulum, 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 centrosomes (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. 25), or consisting of material 

 ingested by the cell. 



In the protoplasm, vacuoles are sometimes found, and 

 from a study of these vacuoles 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 par- 

 ticles. In some cells vacuoles may appear in the process 

 of disintegration. 



14 



