THE PHYSICAL BASIS OF LIFE 



19 



plasm, but it is quite evident that such a term stands for no essen- 

 tial morphological part of the cell, and we have no absolute criterion 

 to distinguish between some granules which are regarded as meta- 

 plasmic in nature and others which are ordinarily considered active 

 elements of the cytoplasm. But there are various undoubtedly 

 active bodies besides the nucleus in the cytoplasm. Chief among 

 these are the centrosome which plays an essential part in cell 

 reproduction, and the plastids, mitochondria, and golgi rodies 

 which apparently are the seat of various special physiological 

 activities. (Fig. 10.) 



The cytoplasm, since it forms the general groundwork, is that 

 part of the cell which comes most closely into relations with the 

 environment, and accordingly near the surface it is frequently 



Golgi bodies 

 Centrosome 



2 



Nucleolus 

 Linin 



Chromatin 



Plastid 



Vacuole 



Cell wall 



Plasma-membrane 



Ectoplasm 



Endoplasm 



Hyaloplasm 



Mitochondria 



Metaplasm 



Fig. 10. — Diagram of a cell. 



modified somewhat in texture and consistency so that a definite 

 outer region, or ectoplasm, may be distinguished from an inner, 

 or endoplasm. The ectoplasm is limited externally by a plasma- 

 memrrane just beneath the cell wall. The plasma-membrane is 

 certainly a part of the living cytoplasm, while the cell wall must 

 be regarded as non-living, though in many cases it is a direct 

 transformation of the living material which grows and plays, 

 in connection with the plasma-membrane, an important part in 

 controlling the flow of matter and energy to and from the cell and 

 its surroundings. 



