CHAPTER XIII 

 ATYPICAL MITOSIS AND OTHER NUCLEAR PHENOMENA 



In the foregoing chapters attention has been restricted largely to 

 typical nuclei and nuclear division as these are observed in the great 

 majority of plant and animal tissues. In this chapter will be reviewed 

 briefly certain additional modes of division and other phenomena which, 

 though in many instances abnormal, have served to broaden our knowl- 

 edge of the nucleus and its significance. 



Amitosis.^ — In amitotic, or direct, nuclear division the nucleus 

 simply constricts and separates into two portions while in the metabolic 

 condition, no condensed chromosomes or achromatic 

 figure being formed (Fig. 107). When the portions are 

 very unequal in size, the process is called "nuclear bud- 

 ding"; when they are more than two in number it is 

 referred to as "nuclear fragmentation." Such nuclear 

 divisions are not followed by cytokinesis; the cells thus 

 come to have two or more nuclei. Amitosis was at one 

 time looked upon as the prevailing mode of nuclear divi- 

 sion, mitosis even being somewhat exceptional, but the 

 true state of affairs, so far as higher organisms are con- 

 cerned, has turned out to be quite the reverse, mitosis 

 occurring almost universally and true amitosis in com- 

 paratively few well-authenticated cases. It is now 

 evident that binucleate cells, constricted or fusing nuclei, 

 and especially the results of aberrant mitosis have often Amitosis in inter- 

 been hastily construed as amitotic division, and it is in ^°^® °^ Chara. 

 the light of this fact that many reports of amitosis should be read.^ 



The amitotic phenomena so frequently reported in cells with a dis- 

 tinctly nutritive function, such as certain gland cells of animals and 

 tapetal, antipodal, and endosperm cells of angiosperms, have been 

 thought to substantiate the widely adopted hypothesis that amitosis 

 aids in metabolism by increasing the nuclear surface. The supposed 

 amitosis in tapetal cells has been shown in a number of instances to be 

 aberrant mitosis (Bonnet), but Tischler considers it probable that true 



1 Bonnet (1912), Beer and Arber (1919), Schtirhoff (1920), Kater (1927a). For 

 reviews of the literature pertaining to amitosis and irregular mitosis in plants, see 

 Tischler (1921-1922, Chap. VII) and Schurhoff (1926). For the literature on amitosis 

 in animals, see Conklin (1917), Nakahara (19186), and Wilson (1925, p. 214). 



183 



Fig. 107.— 



