310 



ELEMENTS OF BIOLOGY 





For convenience in this account the two members of a pair o£ 

 autosomes are designated according to their parentage, as p and m 

 (p having been inherited from the male parent and m from the fe- 

 male parent). Thus at the end of the prophase in mitosis in the 

 human female there are at the equator of the spindle twenty-three p 

 and twenty-three m chromosomes, that is, autosomes, and one pX 

 and one mX chromosome. In the male the arrangement is twenty- 

 three p and twenty-three m chromosomes, one mX and one pY 



chromosome. 



The prophase is followed without 

 pause by the metaphase (Fig. 198), 

 which is brief and includes only the 

 lengthwise splitting of each chromo- 

 some into exactly equal halves. If at 

 this time one passed a plane exactly 

 between the members of divided pairs, 

 on each side of the plane would be a 

 somatic number of chromosomes, each 

 one-half the size of the original p or m 

 chromosomes that have appeared during the prophase. 



The Anaphase and Telophase (Fig. 199). The anaphase 

 follows at once. The new p and ?n chromosomes that result from 

 the splitting during the metaphase now move toward the nearest 

 centrosome, each product of the original p and m members going 

 to opposite poles of the spindle. Rod-shaped chromosomes often ap- 

 pear drawn out into a V during this migration. At this time the 

 surface of the cell flattens around an equator exactly opposite the 

 equator of the spindle. Then a groove appears in the cell boundary. 

 The groove becomes deeper as the chromosomes move nearer the 

 centrosomes, and appears to constrict the cell. Sometimes the groove 

 is wide and the dividing cell appears dumb-bell shaped. Presently 

 the cell separates into two daughter cells, the actual division im- 

 mediately preceding the last stage of mitosis, the telophase (Fig. 



Fig. 199. — The anaphase. 



