i86 
STRUCTURE AND LIFE HISTORIES 
substance called chromatin; 1 all these are enclosed by a 
nuclear membrane. In the non-dividing nucleus the 
chromatin is distributed on the linin threads in the 
form of minute granules (Fig. 136). At one of the stages 
preliminary to nuclear division the linin network, with the 
chromatin, becomes transformed into a thickened skein, 
D \ VI S \ON 
FIG. 138. Diagram of a cytological life-cycle, based on a hypothetical 
fern with four chromosomes in the sporophyte. The nuclear phenomena 
are based on those of the thread- worm (Ascaris). Each chromosome is 
designated by a characteristic mark so that it may be traced throughout 
the diagram. (After R. F. Griggs.) 
which shortly becomes split into two, throughout its entire 
length. The skein finally becomes divided transversely 
into a number of double chromatin bodies or chromosomes. 
The number of these chromosomes is characteristic, and 
always the same for each species of plant. The nuclear 
1 Because it stains readily when treated with aniline dyes. 
