184 
STRUCTURE AND LIFE HISTORIES 
168. Nature and Method of Reduction. In order 
thoroughly to understand fertilization and reduction one 
must have a knowledge of the structure and behavior of 
the nucleus in cell-division and cell-fusion. This subject 
FIG. 136. Diagram illustrating various stages of indirect nuclear 
division (mitosis). A, resting nucleus of the mother-cell; B, formation 
of nuclear skein or spirem; C, longitudinal splitting of the spirem; D, the 
chromosomes (four in number) have been formed by the transverse seg- 
mentation of the spirem; E, chromosomes arranged on the equator of the 
nuclear spindle; F and G, early and late anaphase, the chromosomes moving 
to the pales of the spindle;/?, formation of daughter spirems; 7, resting 
stage of the two daughter-cells. 
is too difficult and too extended to be thoroughly treated 
in an introductory study, but the salient facts are as 
follows. The nucleus of all cells comprises at least four 
substances: nuclear sap, a threadwork of linin, and a 
