CELLS AND TISSUES 



41 



The mitotic process is a continuous one, but for descriptive pur- 

 poses biologists have divided it into four stages: prophase, metaphase, 

 anaphase and telophase (Fig. 3.5). Between mitoses a cell is said to be 

 in the resting stage. It is difficult to visualize from a description or 

 diagram of mitosis, or from examining a fixed and stained slide of cells, 

 just how active a process cell division is. Motion pictures made by phase 

 microscopy reveal that a cell undergoing division bulges and changes 

 shape like a gunny sack filled with a dozen unfriendly cats. 



Prophase. The chromatin threads condense and form visible 

 chromosomes, which appear as a tangled mass of coiled threads within 

 the nucleus. Early in prophase the threads are stretched maximally so 

 that the individual chromomeres are visible. Later in prophase the 

 chromosomes shorten and thicken and the chromomeres lie so close 



Figure 3.5. Mitosis in a cell of a hypothetical animal with a diploid number of six 

 (haploid number = 3); one pair of chromosomes is short, one pair is long and hooked, 

 and one pair is long and knobbed. A, Resting stage. B, Early prophase, centriole divided 

 and chromosomes appearing. C, Later prophase, centrioles at poles, chromosomes short- 

 ened and visibly double. D, Later prophase, nuclear membrane dissolved, spindle pres- 

 ent. E, Metaphase, chromosomes arranged on the equator of the spindle. F, Anaphase, 

 chromosomes migrating toward the poles. G, Telophase, nuclear membranes formed; 

 chromosomes elongating; cytoplasmic division beginning. H, Daughter cells, resting 

 phase. 



