26 



HISTOLOGY 



with the advance of mitosis in such a manner that it may take part in 

 the formation of the transverse cell- wall or cell- membrane. 



In most animals and in some plants 

 the spindle of mitosis has at each pole 

 an aster with its cytoplasmic rays, 

 archoplasm and centriole. A spindle 

 with its asters is termed an amphi- 

 aster. There is usually one spindle 

 arising from a single nucleus. In 

 certain plant tissues and in pathologi- 

 cal tissues very complicated spindles 

 arise with more than two poles. In 

 the nurse cells of magnolia, for ex- 

 ample, tripolar spindles are met with 



FIG. 22. Tripolar mitotic figure from (Fig. 22). Figure 23 shows a multi- 

 nurse in pollen sac of Magnolia. ^^ spmdle from 



The spindle arises in many Protozoa and 



Protophyta within the nuclear membrane. Its 



origin within the nucleus has been described in 



certain plants such as Erysiphe and Fucus (Fig. 



24). Until recently the aster was held to be as 



constant and permanent a feature of the cell as 



the chromatin. Recent evidence makes this 



quite unlikely. The aster with its rays, archo- 

 plasm and centriole, may as completely vanish 



as the spindle, to reappear when mitosis again 



ensues. 



This complex nuclear activity is usually 



accompanied by a division of the cytoplasm 



of the cell. It occurs, however, in certain cells 



where there seems to be a demand 

 for greater nuclear surface without 

 cytoplasmic division. This results in 

 multinuclear cells. We have examples 

 of this in the giant marrow cells and in 

 the male nurse cells of some plants. 

 In the nurse cells of the anthers of 

 Magnolia, for instance, there is an 



FIG. 24. Mitotic figure in Erysiphe. Spin- intermediate example, where we some- 

 die formed inside of nuclear membrane t i mes fi n( J mitosis with Cytoplasmic 

 (n.m.) After HARPER. . . J r 



division, and at other times without 

 cytoplasmic division (see Fig. 383). 



An Example of a Mitosis without Centrosomes. For this mitosis let 



23. v-ancer cell from 

 man. Multiple division by 

 many centers. 



