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; , 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; H, 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 



