16 GERM-CELL CYCLE IN ANIMALS 



surrounded by a number of radiating astral rays, and 

 a spindle which lies between them. The chromo- 

 somes lie in the equatorial plate (ep). 



(b) During the second stage, the metaphase, the 

 chromosomes split in such a way that each of their 

 parts contains an equal amount of chromatin (E, ep). 

 As we shall see later, this is one of the most significant 

 events that takes place during mitosis. 



(c) During the anaphase (F) the chromosomes 

 formed by splitting move along the spindle fibers 

 to the centrosomes. As a result every chromosome 

 present at the end of the prophase (D) sends half of its 

 chromatin to either end of the spindle. The mechan- 

 ism that brings about this migration is as yet some- 

 what in question. Fibers are usually left between 

 the separating chromosomes ; these are known as 

 interzonal fibers (F, if). 



(d) The telophase (G, H) is a stage of reconstruction 

 from which the nuclei emerge in a resting condition ; 

 the chromatin becomes scattered through the nucleus, 

 which is again enveloped by a definite membrane 

 (H) ; the centrosome divides and, with the centre- 

 sphere, takes a position near the nucleus. Finally 

 the cycle is completed by the constriction of the cell 

 into two daughter cells. 



There are a number of differences between the 

 sort of mitosis just described and that which occurs 

 during the maturation of the egg and spermatozoon ; 

 these and certain other phases of cell division will 

 be considered in their appropriate places in succeed- 

 ing chapters. 



