Chap. 3 LIVING MATTER AND CELLS 41 



attachment to the spindle. This "owes its existence to a particular gene lying 

 at that point" (Muller, 1947). When it is at the center the chromosome is 

 V-shaped with the tip of the V in contact with a line of the spindle. Some- 

 times it is fairly near the end and the chromosome then hangs J -shaped on 

 the spindle, or, if very close to the end, it is rodlike. During the early part 

 of the metaphase the centromeres are apparently repelled from the poles 

 of the spindle and moving toward the equator they draw their chromosomes 

 with them. There, all the chromosomes become arranged exactly half-way 

 between the poles of the spindle at the midplane in an equatorial plate. The 

 chromosome and its duplicate are still in contact (Fig. 3.8), 



Anaphase (Separation). Each chromosome and its duplicate begin to 

 separate always starting at the centromeres which are responsive to the forces 

 of the attraction of the spindle. The members of each pair of chromosomes 

 gradually draw apart until they become entirely separated and each one 

 moves toward the nearer pole. During this journey the centromere is always 

 in front, pointed toward the pole (Fig. 3.8). Late in the anaphase the 

 chromosomes are in two identical groups, one at each end of the spindle. 

 In each group the chromosomes of the parent cell with their genes are all 

 represented. 



In animal cells the division of the cytoplasm starts from the outside and 



Fig. 3.8. Diagrams showing changes in the nucleus during the reproduction of a 

 cell by mitotic division such as occurs in every cell of a growing body, or in parts of 

 the adult, except in the later divisions of maturing sex cells. /. Part of an egg 

 shortly after its fertilization. Three chromosomes (black) represent the inheritance 

 from the male parent, and three (in outline) the inheritance from the female 

 parent. The descendants of these six chromosomes occur in all the cells of the new 

 individual. The star-shaped centrosome is a center of force. 2. Interphase or "rest- 

 ing stage" with the chromosomes uncoiled in threads so ensnarled that individual 

 chromosomes cannot be identified except with great difficulty. 3. Early prophase. 

 Each thread has doubled and now consists of two identical strands, thickened by 

 means of the ultra-fine coiling of the strands. The centrosome has divided and there 

 are now two centers of force. 4. Late prophase. The chromosomes are mates, 

 shortened and lying side by side. Every one of the thousands of genes contained 

 in one is duplicated in the other. The centrosomes are moving to opposite sides of 

 the nucleus and the nuclear membrane is dissolving. 5. Early metaphase. Lines of 

 force from the centrosomes have become attached at given points (centromeres) 

 to the respective mates, called identical chromosomes. This has forced them into 

 positions on an equatorial plane half way between the centrosomes. 6 and 7. Late 

 metaphase and late anaphase. The apparent lines of force exert a pull on the 

 centromeres, thus separating the identical chromosomes, and drawing the respective 

 mates toward the opposite ends of the spindle formed by the lines of force. 

 8. Division completed. Interphase. (Telophase omitted.) The two identical groups 

 of chromosomes are pulled near to the centrosomes and ceil membranes separate 

 the cell body into halves. The fine coils of the chromosomes unwind in threads 

 similar to those in 2. This process occurs in the telophase stage not shown here. 

 With the attainment of two new cells in the interphase stage, the reproduction is 

 completed. 



