CELLS IN DIVISION 



in the state of hydration of the matrix during the chromosome cycle 

 play a part in the arrangement of the spirals of the chromonemata. 



There is increasing evidence of chromosomes at all stages of nuclear 

 division containing at least two chromonemata. This subject is reviewed 

 by Kaufman.^11 The crucial stage is that of the daughter chromosomes 

 at anaphase ; in Tradescantia and Trillium the anaphase chromosomes in 

 somatic metaphases are four-stranded (Nebel and Ruttle"^). In 

 animal material, a double-stranded condition of the somatic chromo- 

 somes had been described in a number of Orthoptera (Mickey"*) 

 (Figure 32) ; in the giant ganglion cells of the embryo of Drosophila 



,m ^: 



70ft ^ 



Figure 32 The two daughter haploid 

 groups of chromosomes during anaphase 

 in the second maturation division of the 

 male germ cells of Romalea microptera. The 

 x-chromosome (4) is seen in both. Each 

 monad is comprised of two twisted 

 chromatids: the internal spirals are 

 drawn in only part of the figure X1500. 

 From Mickey"* {By courtesy, American 

 Naturalist) . 



(Kaufman^i^); and in one vertebrate, namely the amphibian Amby- 

 stoma, by Creighton.^^* In spleen cells of the mouse in tissue culture 

 (Fell and Hughes^^') phase-contrast photomicrographs have revealed 

 that the stage of apparent coalescence of the daughter chromosomes in 

 anaphase is succeeded by one in which the whole mass loosens to reveal 

 fine threads much smaller in diameter than the metaphase chromo- 

 somes (Plates IX and X (13) ). The bearing of the evidence of mul- 

 tiple nature of chromosomes on the question of the period of their 

 duplication in relation to mitosis is summarized by Manton:^^^ 



If a chromosome is at all times multiple, 'splitting' in any literal sense may never 

 occur at all. The multiplication of unit strands viewed as a chemical problem may, 

 but need not, involve fission of the small sub-microscopic threads. Chromosome 

 bipartition, on the other hand, may possibly only entail the spatial separation of 

 bundles of strands each of which has long had an individual existence. 



The alternative view is that the chromosome splits at a definite 

 point in the division cycle. Darlington^^^ maintains that this is before 

 the prophase of mitosis, but the meiotic prophase is believed to begin 

 precociously in advance of this. Evidence bearing on the mitotic aspect 



93 



