INTRODUCTION 15 



ranged in the form of a long thread or spireme (B). At the same 

 time the centrosomes move apart (A, c ; B, a). The radiating 

 lines that appear about them (B) later give rise to a spindle (C). 



A B 



FIG. 4. Mitosis. Diagrams illustrating mitotic cell division. (From 

 Wilson.) A, resting cell; B, prophase showing spireme and nucleolus within 

 the nucleus and the formation of spindle and asters (a); C, la<:er prophase show- 

 ing disintegration of nuclear membrane, and breaking up of spireme into 

 chromosomes; D, end of prophases, showing complete spindle and asters with 

 chromosomes in equatorial plate (ep); E, metaphase each chromosome splits 

 in two; F, anaphase the chromosomes are drawn toward the asters, if = 

 interzonal fibers; G, telophase, showing reconstruction of nuclei; H, later 

 tclophase, showing division of the cell into two 



While this is going on the nuclear membrane generally disin- 

 tegrates and the spireme segments into a number of bodies called 

 chromosomes (C); these take a position at the equator of the 

 spindle, halfway between the centrosomes (D, ep). The stage 

 shown in Figure 4, D, is known as the am pinaster; at this time 



