CAUSES OF VARIATION 



Fig. 20. Diagrams showing the prophases 

 of mitosis 



, resting cell with reticular nucleus and true nucleolus : at c the attraction sphere containing 

 two centromoses. B, early prophase : the chromatin forming a continuous spireme, 

 nucleolus still present; above, the amphiaster (a). C, D, two different types of later 

 prophases : C, disappearance of the primary spindle, divergence of the centrosomes to 

 opposite poles of the nucleus (examples, some plant cells, cleavage stages of many eggs) ; 

 D, persistence of the primary spindle (to form in some cases the "central spindle"), 

 fading of the nuclear membrane, ingrowth of the astral rays, segmentation of the spireme 

 thread to form the chromosomes (examples, epidermal cells of salamander, formation of 

 the polar bodies). , later prophase of type C: fading of the nuclear membrane at the 

 poles, formation of a new spindle inside the nucleus ; precocious splitting of the chromo- 

 somes (the latter not characteristic of this type alone). F, the mitotic figure established ; 

 </, the equatorial plate of chromosomes. After Wilson 



