ISO 



FOOT-NOTES TO EVOLUTION. 



fluid substance termed protoplasm or cytoplasm. By 



this term is understood not a homogeneous, definite 



chemical substance, but rather an organ- 

 The protoplasm. . , . ^. , ^ , , 



ized association of extremely complex 



organic compounds, belonging mainly to the group of 

 proteids. The majority of these substances are but lit- 

 tle understood as yet, nor is this at all surprising when 

 we reflect that the living cell is the theater of constant 

 changes, both synthetic and analytic, and that the dead 

 protoplasm subjected to the chemist's analyses is no 

 longer protoplasm, but has suffered profound transfor- 

 mations in passing from the living to the lifeless con- 

 dition. 



In its simplest form the cell is an approximately 

 spherical, viscid, granular structure in which oftentimes 

 there may be made out, in the living state, more solid 

 substances in the form of threads or networks of delicate 

 filaments. This threadwork is probably made up of 

 rows of granules, and varies in arrangement in different 

 cells and in different parts of the same cell. Within 

 the meshes of this reticulum is inclosed a clearer fluid- 

 like portion — the cytolymph or hyaloplasm. 



Inclosed in the cytoplasm lies a spherical or ovoidal 

 body — the nucleus — set off from the rest of the cell by 

 a more or less distinct boundary mem- 

 The nucleus. brane. This structure is of constant 

 occurrence in all cells, and plays an extremely impor- 

 tant part in their life history, forming apparently the 

 controlling centre of the constructive processes in growth 

 and multiplication. Its significance will be better un- 

 derstood further on in connection with the problems of 

 cell development and heredity. 



In chemical and physical properties the nucleus 

 differs markedly from the rest of the protoplasm. When 

 studied in detail by the aid of suitable reagents and 



