OTHER MODES OF NUCLEAR DIVISION 



211 



though the divisions giving rise to the ascospore nuclei have been shown 

 to be mitotic in certain cases. (See Guilliermond 1920.) Amitosis 

 was once believed to be the normal mode of nuclear division, mitosis 

 being looked upon as very exceptional. The true condition, so far as 

 higher organisms are concerned, has turned out to be quite the reverse: 

 it is evident that amitosis occurs frequently in certain kinds of cells, but 

 the mitotic method of division has been found to be 

 almost universal. 



What the physiological significance of amitosis 

 may be is not well known. It was once suggested 

 (Chun 1890) that it aids the processes of metabolism 

 by increasing the nuclear surface in the cell, since 

 it is of such frequent occurrence in cells with a dis- 

 tinctively nutritive function. This view has recently 

 been restated by Nakahara (1917) as a result of his 

 work on the larva of Pieris. l The most generally held 

 opinion regarding amitosis in the higher organisms 

 was for many years that expressed by Flemming 

 (1891), namely, that it represents a degeneration 

 phenomenon or aberration of some kind, which would 

 explain why it is so often found in degenerating and 

 pathological tissues. In the words of vom Rath 

 (1891), "when once a cell has undergone amitotic 

 division it has received its death-warrant; it may indeed continue to 

 divide for a time by amitosis, but inevitably perishes in the end." 



That the view of vom Rath must be modified has been indicated by 

 the results of a number of investigations. For instance, Pfeffer (1899) 

 and Nathansohn (1900) found that if Spirogyra filaments are placed in a 

 ^ to 1 per cent solution of ether the nuclei divide by amitosis only, and 

 that when the filaments are returned to pure water the mitotic method 

 of division is resumed, with no evidence of degeneration. Haecker, how- 

 ever, working on the eggs of Cyclops, came to view such artificially in- 

 duced behavior not as true amitosis but rather as a much modified 

 mitotic division, which he termed "pseudoamitosis." Other cytolpgists 

 observed nuclear divisions that seemed intermediate in character between 

 mitosis and amitosis (Dixon in the endosperm of Fritillaria, 1895; Sargant 

 in the embryo sac of Lilium, 1896; R. Hertwig in Actinosphcerium, 1898; 

 Buscalioni in the endosperm of Corydalis, 1898; and Wasielewski in the 

 roots of Vicia faba, 1902, 1903). Hertwig accordingly concluded that 

 mitosis and amitosis are separated by no sharp boundary line, but are 

 connected by an unbroken series of transition stages. 



1 In a second paper (1918) Nakahara gives a convenient review of the literature 

 of the subject. 



FIG. 75. Amitosis 

 in internodal cell of 

 Chara. X 413. 



