THE WONDERS OF LIFE 



are stored within the living cytoplasm, the latter thrust 

 out from it. 



Internal plasma-products of common occurrence are 

 the microsomata, very small and opaque particles which 

 are generally regarded as products of metabolism. They 

 consist sometimes of fat, sometimes of derivatives of 

 albumin, sometimes of other substances of which we do 

 not know the chemical composition. The same may be 

 said of the large and variously-colored pigment-granules, 

 which are very common and determine the color of 

 tissues. Also very common in the cytoplasm are large 

 accumulations of fat in the shape of oil-globules, fat- 

 crystals, etc., besides other crystals of a very different 

 sort, partly organic crystals (for instance, albuminous 

 crystals in the aleuron - granules of plants), partly 

 inorganic crystals (for instance, of oxalic-acid salts in 

 many plant-cells, of calcareous salts in many animal- 

 cells). The watery cell-sap (cytolymph) plays an impor- 

 tant part in many of the larger cells. It is formed by the 

 accumulation of fluid in the cytoplasm, and is found in 

 its frothy structure. The large empty spaces which it 

 forms are called vacuoles, with very regularly disposed 

 alveoles. When the cell-sap gathers in great abundance 

 within the cell, we get the large vesicular cells which 

 are found in the tissues of the higher plants, the carti- 

 lages, etc. 



As external excretions of the living cytoplasm that 

 have acquired some importance, especially as protective 

 organs, in the majority of cells, we have first of all the 

 cell -membranes, the firm capsules or protective skins 

 which enclose the soft cell-body, like a snail in its house. 

 In the first period of the cell-theory (i 838-1 859) such an 

 integument was ascribed to all cells, and often regarded 

 as their chief constituent; but it was discovered after- 

 wards that this protective skin is altogether wanting in 

 many (especially animal) cells, and that it is not found 



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