I 6 COLLEGE ZOOLOGY 



all of the machinery concerned in mitosis is present. There are 

 two asters, each consisting of a centrosome surrounded by a num- 

 ber of radiating astral rays, and a spindle which lies between 

 them. The chromosomes 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 ana phase (F) the chromosomes formed by split- 

 ting 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 

 mechanism that brings about this migration is as yet somewhat 

 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 throughout the nucleus, which is again enveloped by 

 a definite membrane (H) ; 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 daugh- 

 ter cells. 



CHROMOSOMES. - - Every species of animal has a definite 

 number of chromosomes that appear when the cells of its 

 body undergo mitosis. Thus sixteen are characteristic of 

 the cells of oxen, guinea pigs, and man; the grasshopper has 

 twelve; and the brine shrimp (Artemia) one hundred and sixty- 

 eight. An even number of chromosomes is characteristic of 

 most animals, but recent researches have demonstrated that 

 some forms, particularly the males of insects, have an odd 

 number. The chromosomes are considered by most zoologists 

 to be the bearers of hereditary qualities from parent to 

 offspring. 



In concluding this account of cell division two points are 

 worthy of special emphasis. First, with regard to the continuity 



