100 



EVOLUTION AND GENETICS 



Fission. Single-celled organisms often reproduce by a super- 

 ficially similar process called fission which also produces two similar 

 individuals at the expense of parental loss of individuality, but 

 this process foreshadows a more complex type of reproduction 

 called mitosis. It is said by some authorities to be a true mitosis. 



A y^ "-\ B /r-^"~7>x C 



Fig. 61. — Typical stages of mitosis in which the chromosome number is 

 assumed to be eight. A, beginning of prophase: chromatin in a reticular 

 form, centrosome divided and astral fibers formed about it; B, early pro- 

 phase: chromatin in spireme, centrosomes moving apart and spindle form- 

 ing between them; C and D, later prophase: chromosomes forming and 

 remainder of nucleus breaking down; E, metaphase: the chromosomes ar- 

 ranged in the equator of the spindle and split longitudinally; F, G, anaphase: 

 the chromosomes migrating toward the centrosomes; H, telophase: a 

 gradual return to the original state of the nuclear constituents and centro- 

 somes, accompanied by constricti'on of the cytoplasm; I, the two cells formed 

 at the completion of mitosis. (From Woodruff.) 



Mitosis. This process involves primarily the chromatin of the 

 nucleus (Fig. 61). During mitotic division of animal cells the 

 centrosome divides, the two halves move away from each other, 

 and about each a series of radiating fibers or apparently fibrous 

 structures appears. Between the two a series of connecting fibers 

 develops. The entire structure is not unlike the field of a bar 



