98 



GENERAL BIOLOGY 



chromosomes are laid hold of on all sides, " multi- 

 polar " spindles being formed. These phenomena 

 indicate that the spindle is a complicated piece of 

 mechanism, which like all machines may " go wrong." 



Nature of the Centrosome. The centrosome, 

 from the leading part it appears to play in cell 



division, has been the 

 object of very careful 

 research. It has been 

 held by some that it 

 is a permanent organ 

 of the cell, but this 

 seems not to be the 

 case. Not only is it 

 normally absent in 

 many dividing cells, 

 but there is no ques- 

 mitoses. tion but that it is 

 formed anew in suc- 

 ceeding cell divisions, 

 most often in the cyto- 

 plasm. Centrosomes, 



moreover, have been caused to develop by chemical 

 reagents in enucleated fragments of unfertilized eggs 

 after the true egg centrosome and spindle has 

 developed in another part of the same egg. 



The mechanical basis of the mitotic spindle has 

 been difficult to get at, because practically the only 

 means of studying cell structure is by killing, fixing, 

 hardening, sectioning, and staining cells and tissues, 



FIG. 34. Abnormal 

 Four-poled spindle in a developing 

 sea-urchin's egg after poisoning with 

 quinine. The spindle x-y lacks its 

 share of the chromosomes. (Hert- 

 wig.) 



