76 PRINCIPLES OF ANIMAL BIOLOGY 



later nucleoli appear. The achromatic figure begins to fade away. 

 Traces of the spindle, however, may sometimes be seen connecting the two 

 nuclei, but pinched together in the middle by the constriction of the 

 cell membrane. The astral rays about the ccntrosomes disappear. The 

 centrosome of each daughter cell may divide at this time in certain species 

 or it may remain undivided until about the time of the next division of 

 the cell. In some species it may entirelj^ disappear only to reappear at 

 the beginning of the next mitosis. In cells of the flowering plants a 

 centrosome does not exist. 



Reconstruction may now be said to be complete (Fig. 44). The 

 daughter cells, however, are only half the size of the mother cell from which 

 they arose. There succeeds a period of growth which is followed in 

 turn by another division of the cell. The process of cell division is re- 

 peated most frequently in young and rapidly growing animals but it 

 occurs at all times during the life of the individual. 



VARIATIONS IN MITOSIS 



In many cells of animals and plants mitosis differs in unessential 

 features from the generalized process described above. These variations 

 may concern the achromatic figure, the chromatic figure, the time rela- 

 tions of certain parts of the process, or the mode of division of the cyto- 

 plasm. 



Achromatic Figure. — The centrosome is usually demonstrable in 

 dividing animal cells if proper staining methods are employed, but pains- 

 taking research has now conclusively shown that cells of higher plants 

 do not possess centrosomes. The difference is shown in Figs. 45 

 and 46 which illustrate animal cells with centrosomes, and Figs. 47 and 

 48 representing plant cells in mitosis without centrosomes. Centro- 

 somes are remarkably well shown in the dividing germ cells of Ascaris. 

 When the centrosome is present in the cell its division usually precedes 

 the formation of a spireme. Sometimes, indeed, the division of the cen- 

 trosome occurs during the telophase of one cell division in preparation 

 for the next division. The movement of the centrosome to the opposite 

 poles of the nucleus frequently occurs before rather than during the 

 formation of the spireme. 



The origin of the spindle is subject to some variation. In cells of 

 some species, the salamander for example, the spindle originates outside 

 of the nucleus and its fibers penetrate into the nucleus upon the disso- 

 lution of the nuclear membrane, while in other species the original 

 spindle fibers of the amphiastcr disintegrate and new ones arise from 

 material within the nucleus, possibly from the linin fibers. In certain 

 species of Protozoa (Fig. 40) and in some other animals, among them 

 certain arthropods and rotifers, the spindle is formed inside the nuclear 

 membrane. 



