PLASM 



of parts is extremely great in the cytoplasm. When we 

 wish to arrange them in a few large groups from a 

 general point of view, we may distinguish the active 

 plasma-formations from the passive plasma-products; 

 the former are due to a chemical metamorphosis of the 

 living plasm, the latter lifeless excretions from it. 



Under the head of plasm-formations, or products of 

 differentiation in the cytoplasm, we comprise all forma- 

 tions that are due to partial metamorphosis of the living 

 cell-body — not lifeless excretions from it, but living parts 

 of its substance, undertaking special functions, and 

 therefore chemically and morphologically differentiated 

 from the primary cytoplasm. One of the commonest dif- 

 ferentiations of this kind is the separation of the firm 

 hyaline skin-layer (hyaloplasm) from the softer granular 

 marrow-layer (polioplasm) ; though the two often pass 

 into each other without clear limits. In most plant-cells 

 special granules of plasm, mostly globular or roundish, 

 are developed, called tropJioplasts, and these undertake 

 the work of metabolism. To this class belong the amylo- 

 plasts, which produce starch (amylum), the chloroplasts 

 or chlorophyll-granules which form the green matter 

 (chlorophyll) in the leaf, and the chromoplasts which 

 form color-crystals of various sorts. In the cells of the 

 higher animals the myoplasts form the special contractile 

 tissue of the muscles, and the neuroplasts the psychic 

 tissue of the nerve-matter. On the other hand, the dis- 

 tinction between the body-plasm (somoplasma) and the 

 germ-plasm (germoplasma), which serves as the base of 

 Weismann's untenable theory of the germ-plasm (cj. 

 chapter xvi.), is purely hypothetical and without direct 

 observation to support it. 



The infinite variety of parts of the cell which arise as 

 excretions of the living active cytoplasm, and so must be 

 regarded as lifeless plasma-products, may be divided into 

 two chief groups — internal and external. The former 



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