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INTRODUCTION TO CYTOLOGY 



somes have reached polar positions. The centrosomes, surrounded by 

 asters, remain at the poles of the achromatic figure during metaphase, 

 anaphase, and telophase, and after mitosis has been completed they may 

 disappear or remain through the resting stage to function in the next 

 mitosis. (See p. 78.) 



The division of the cell following nuclear division is commonly 

 brought about in animals by the formation of a cleavage furrow, which 

 grows inward from the periphery as described in the following chapter, 

 rather than by the formation of a wall on the spindle fibers as in 

 plants. 



Fig. 49. — Diagram of a typical case of somatic mitosis in animals. 



Although the two above points serve in general to distinguish mitosis 

 in animals from that in plants, the distinction is not a sharp one: cen- 

 trosomes are regularly present in the cells of many lower plants, while 

 cytokinesis by furrowing also occurs in certain cases, as will later be 

 shown. The essential point to be borne in mind is that the significant 

 feature of mitosis — the division of the chromatin and its distribution to 

 the daughter nuclei — is fundamentally the same in both plants and 

 animals. 



The relative duration of the various phases of mitosis has been studied 

 in a few cases. As an example may be taken the observations of M. and 

 W. Lewis (1917) on the mesenchyme cells of the chick growing in tissue 

 cultures. These investigators summarize the researches of others upon 

 the subject and give the following figures for the chick cells: prophase, 

 5 to 50 minutes, usually more than 30; metaphase, 1 to 15, usually 2 

 to 10; anaphase 1 to 5, usually 2 to 3; telophase up to cytokinesis, 2 to 13, 

 usually 3 to 6; telophasic reconstruction of daughter nuclei, 30 to 120; 

 total, 70 to 180 minutes. 



