THE CELL AND CELL DIVISION 



57 



in these divisions is very complex; that there is here only one-half the 

 number of these bodies formed in the cells of the somatic tissues; that 

 mitoses may occur without an intervening interkinesis ; that in the 

 case of the egg cell the division may be of extreme inequality, so that 

 one of the daughter cells is like an extremely small bud, although with 

 respect to nuclear structure the two cells are alike ; and that in certain 

 special mitoses one or more of the chromosomes may fail to divide and 

 therefore may be unrepresented in one of the daughter cells. 



Among the higher plants an important characteristic is the absence 

 of definite centrosomes and asters, although these structures are 

 normally present among the lower plants. The absence of centrosomes 

 results in the formation of a characteristically blunt or truncated 



FIG. 25. A. Multipolar spindle in spore-mother-cell of Equisetum. From 

 Wilson, "Cell," after Osterhout. B. Intranuclear spindle in the oocyte of the 

 Copepod, Canthocamptus staphylinus. From Hegner. X 850. 



spindle in most of the higher plants. In some animals the spindle is 

 rather truncated also, but this is usually found to be in reality multi- 

 polar, composed of many small bundles of spindle fibers terminating 

 in a row of centrosomes or centrosome-like bodies (Fig. 25, A). In the 

 tissue cells of most animals the asters are relatively small, though the 

 spindle remains large and distinct; in a few cases it seems that even in 

 animal cells division may be effected in the absence of centrosomes. 



Many significant modifications of mitosis occur among the Protozoa, 

 where we find certain conditions which seem to offer suggestions as to 

 the evolution of the mitotic figure and process, as well as of some of the 

 chief cell organs themselves. Among these forms the process of divi- 

 sion is often complicated through its being at the same time the essential 

 step in reproduction, rather than merely a step in or condition of differ- 

 entiation, as in the Metazoan. Thus the process of budding or gemma- 

 tion is essentially an unequal cell division; and in brood ("spore") for- 

 mation we see a form of cell division in which the nucleus divides many 



