150 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- 
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 
amore or less distinct boundary mem- 
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 
The protoplasm. 
The nucleus. 
