THE CELL AND THE CELL THEORY 31 



halfway between the centrosomes (E). The stage shown in 

 Figure 6, F, is known as the amphiaster ; at this time all of 

 the machinery concerned in mitosis is present. There are two 

 asters, each consisting of a centrosome surrounded by a number 

 of radiating astral rays, and a spindle which lies between them. 

 The chromosomes lie in the equatorial plate (ep). 



During the second stage, the metaphase, the chromosomes split 

 in such a way that each of their parts contains an equal amount 

 of chromatin (G). As we shall see later, this is one of the most 

 significant events that takes place during mitosis. Often the 

 chromosomes split before they have assumed an equatorial posi- 

 tion (Fig. 6, E) ; other minor variations also occur. 



During the anaphase (Fig. 6, H), 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 

 (F) sends half of its chromatin to either end of the spindle. The 

 mechanism that brings about this migration is as yet somewhat 

 in question. Fibers are usually left between the separating 

 chromosomes; these are known as inter zonal- fibers (H,if). 



The telophase (Fig. 6, I, J) is a stage of reconstruction from 

 which the nuclei emerge in a resting condition; the chromatin 

 becomes scattered throughout the nucleus, which is again envel- 

 oped by a definite membrane (J) ; the centrosome divides and, 

 with the centrosphere, takes a position near the nucleus. Finally 

 the cycle is completed by the constriction of the cell into two 

 daughter cells (I, J) each of which is in condition to carry on the 

 regular metabolic processes until it in turn becomes ready to 

 divide. 



The origin of the structures that take an active part in mitosis 

 is not definitely known. Only the centrosomes are represented 

 in the resting cell; these usually arise from the division of pre- 

 existing centrosomes, but in certain cases they are wholly absent 

 (higher plants), or their existence is questionable (Protozoa). 

 The chromosomes are formed directly by a condensation of chro- 

 matin. The origin of the spindle fibers is variable. Sometimes 



