THE STRUCTURE OF THE CHROMOSOMES 133 



some in the telophase develops an endogenous spiral thread which persists 

 through the interphase and emerges from the chromosome in the following 

 prophase. The achromatic material is not continuous from one nuclear 

 generation to the next but is differentiated anew in each telophase as 

 the spiral develops. According to Vejdovsky, who first applied the 

 term "chromonema" to the spiral filament, the latter arises much earlier 

 than stated by Bonnevie: it first develops in the prophase within the yet 

 unsplit antecedent chromonema, and, as the latter splits, the new 

 chromonema becomes broken up into irregular rings or strands which 

 in some undetermined manner form a continuous spiral again in the 

 telophase. The achromatic portion of the chromosome swells and 

 becomes the karyolymph, at the periphery of which the nuclear mem- 

 brane is formed. Such chromonemata have since been observed and 

 discussed by many writers, one of the principal questions at issue being 

 that of the permanence of such filaments through the nuclear cycle. 



That the chromonema is a permanent constituent of the chromosome 

 and not a temporary formation has been rendered evident by recent 

 studies on both somatic cells and sporocytes. In root tips prepared by 

 methods rendering the matrix less stainable the chromatic chromonema 

 can be observed as a contorted or coiled filament lying mostly in the 

 peripheral region of the metaphase or anaphase chromosome. After 

 certain other methods the chromatic matter appears to be more uniformly 

 distributed in the peripheral region of the chromosome, which thus has a 

 tubular appearance.^ In the telophase the chromonema becomes a 

 portion of a new nuclear reticulum, which resolves into its constituent 

 chromonemata again in the next prophase. What is known about the 

 morphology of the individual chromosomes and the characteristic 

 structural patterns exhibited by certain chromonemata (see p. 141) 

 indicates that the prophasic chromonemata are actually the same as 

 those which formed the reticulum in the preceding telophase, although 

 they have undergone temporary alterations in the meantime. 



In large chromosomes it appears that the chromonema in the telo- 

 phase, metabolic stage, and prophase is double as the result of splitting 

 one mitotic cycle in advance of the anaphase in which the halves so 

 formed are to separate. ^ Whether or not this is also true of small 

 chromosomes is unknown. 



The Mitotic Cycle. — Large chromosomes like those just mentioned 

 appear to pass through somatic mitosis essentially as follows (Figs. 73, 74). 



1 E.g., Bonnevie (1908), Zirkle (1928), and Blunt (1932). 



2 K. Schneider (1910) on Salamandra; Dehorne (1911) on Salamandra and Allium; 

 Kaufmann (1925, 1926a6, 1931a) on Tradescantia, Podophyllum, Allium, and Iris; 

 Sharp (1929) on Trillium, Vicia, Podophyllum, Allium, and Tradescantia; Telezynski 

 (1930a6, 1931o6) on Tradescantia and Hcemanthus; Nebel (1932) on Tradescantia; 

 Hedayetullah (1931) on Narcissus; Perry (1932) on Galanthus; F. H. Smith (1932) on 

 Galtonia; Huskins and Huates (unpubl.) on Anthoxanthum. 



