SECTION II. 



The Cell. 



Protoplasm occurs in the animal body as small separate 

 masses of Cells. These vary considerably in size, but in the 

 higher animals, 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 protoplasm. In all higher animals each Cell has a 

 perfectly definite structure. It consists of a mass of proto- 

 plasm, in which is situated a more or less defined body, the 

 nucleus. 



A. Cell Protoplasm. 



1. Structure. — The general characters of protoplasm have 

 been already described (p. 11). In some cells condensation 

 at the surface is marked, and a so-called membrane 

 surrounds them. 



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

 two small spherical bodies, the centrosomes (fig. 3), 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, or consisting of material ingested 

 by the cells. 



In the protoplasm, vacuoles are sometimes found, and 

 from a study of these vacuoles in protozoa, it appears that 



1 The micro-millimetre is the ] o^^jj^jth of a millimetre. 



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