CELL DIVISION 



137 



nc, nucleolus. (From Kellioott, "General Embry- 

 ology," copyright, 1913, by Henry Holt and Co., re- 

 printed by permission.) 



division apparently occurs only in cells that are very specialized, 

 very old, or in some abnormal or degenerating condition (Fig. 75). 



Mitosis. — Mitosis is the typical method of cell division. It is 

 called the indirect method because it involves changes that are 

 more complicated than the simple constriction of amitosis. The 

 process is divided, for purposes of description, into four stages, 

 which are continuous. 

 These are the prophase, 

 the VI ctaphase, the 

 anaphase, and the telo- 

 phase. 



The structure of a 

 typical vegetative cell 

 should be recalled 

 (Fig. 74). Among the 

 earliest changes in the l'^*-- 75.— Amitosis in tendon cells of :i new- 

 prophase of mitosis are ^^^ ™«"««- '^^^^^ Nowikoff.) 



those of the centro- 

 sphere and centrioles. 

 If the cell contains 



only one centriole, this divides and the two halves begin to sepa- 

 rate, passing toward opposite sides of the nucleus (Fig. 76). The 

 centrosphere elongates and gives rise to fine fibers, the spindle 

 fibers, which extend between the centrioles as they migrate, and 

 also to delicate strands, the astral rays, which radiate freely from 

 the centrioles. Collectively, these structures are called the 

 mitotic spindle because of the arrangement of the fibers, the 

 amphiaster, because of the resemblance to a double star, or the 

 achroinatic figure, because these structures do not stain. Within 

 the nucleus the chromatic granules become associated in delicate 

 threads which form what is known as the spireme. It is the forma- 

 tion of these threads that gives the name mitosis (from 7nitos, 

 thread) to this kind of cell division. As the prophase progresses, 

 the chromatin threads condense to form bodies known as chro- 

 mosomes. These chromosomes are constant in number, size, and 

 shape for the cells of any particular species of animal. As migra- 

 tion of the centrioles, formation of the amphiaster, and condensa- 

 tion of the chromosomes progresses, the nuclear membrane and 

 nucleolus disappear. When the nuclear membrane is broken down, 

 the spindle comes to lie in the region of the nucleus and the chro- 



