i86 



STRUCTURE AND LIFE HISTORIES 



substance called chromatin;'^ 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 

 prehminary to nuclear division the linin network, with the 

 chromatin, becomes transformed into a thickened skein, 



D \ VI S \0N 



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 



^ Because it stains readily when treated with aniline dyes. 



J 



