AN INTRODUCTION TO ZOOLOGY 



(i) Mitosis. A cell that is not undergoing division is said 

 to be in a resting condition (Fig. 6, A). When it divides by the 



indirect method, the se- 

 ries of changes may con- 

 veniently be arranged in 

 four stages, known as the- 

 prophase, metaphase, ana- 

 phase, and telophase. The 

 modifications occurring 

 during mitosis are prima- 

 rily concerned with the 

 nucleus and centrosomes, 

 the cytoplasm remaining 

 comparatively passive. 



The prophase is char- 

 acterized by preparatory 

 changes. The chromatin 

 granules that are scat- 

 tered through the nucleus 

 in the resting cell become 

 arranged in the form of 

 a long thread or spireme 

 (Fig. 6,B). At the same 

 time the centrosomes 

 move apart, finally reach- 

 ing opposite sides of the 

 nucleus (B, C, D, E). 

 The radiating lines that 

 appear about them (B) 

 later give rise to a spindle 

 (D). While this is going 

 FIG. 6. Diagrams illustrating mitotic cell on the nuclear membrane 

 division. (From Wilson.) generally disintegrates 



and the spireme segments into a number of bodies called chro- 

 mosomes (D) ; these take a position at the equator of the spindle. 



