THE MITOTIC CYCLE 



bristles do not appear thus, but as fern-like branches (Clark et alii^^^). 

 At the same time, nucleoli begin to form at particular loci on the 

 chromosomes, and this process continues until a surrounding 'cloud' 

 of small nucleoli has developed ; the chromosome loops are then shed 

 into the nuclear sap. The whole complex of chromosomes and nucleoli 

 later contracts away from the nuclear membrane; the chromosomes 

 then shorten and resume their normal diplotene condition. 



It is clear that the lamp-brush cycle is a remarkable example of rapid 

 ribonucleoprotein production, related to the deposition of yolk. The 

 morphological question concerning them is whether the loop-like 

 bristles are to be regarded as pulled out gyres of a coiled chromonema; 

 the answer to this seems to be in the negative, for Dodson^^^ has shown 

 that axis and branches differ in chemical composition, for the fully de- 

 veloped loop is Feulgen negative, while the main axis is Feulgen positive. 

 Moreover Duryee^^^ found that the chromosomes could be greatly 

 stretched by micromanipulation, without ever undoing their lateral loops. 



Thus, with regard to these two large and specialized types of chromo- 

 some, most of the evidence is against the view that the structural unit 

 is a spirally coiled chromonema; on the other hand, in the larger normal 

 mitotic chromosomes their spiral structure is often beyond doubt, 

 though in many somatic chromosomes the most that can be said about 

 their internal structure is that in prophase they consist of apparently 

 chromomeric filaments. When contracted in metaphase, such chromo- 

 somes give the appearance of short cylinders of uniform density. It is 

 usually assumed that their contraction is always due to 'spiralization' 

 which proceeds j&fln j&fljJM with the concentration upon them of nucleo- 

 proteins (Koller^^^). Definite proof of this has not yet been provided. 

 At present, perhaps the most plausible point of view intermediate 

 between the two extremes is the following. Due to the great length of 

 the salivary and oocyte chromosomes they are composed of uncoiled 

 chromonemata, along which are solid aggregates of chromatic material, 

 the chromomeres. Chromosomes of normal length in general have a 

 spiral structure, and may vary in structure along their length both in 

 the arrangement of their coiling, and also in density of deposited 

 material. Ruch^^' claims that the minor spiral of the Tradescantia 

 chromosomes at meiotic metaphase is resolved by means of the phase 

 microscope into a series of chromomeres. On the other hand, Cleve- 

 LAND^"* can clearly see the minor spirals in the living flagellate Holo- 

 mastigotoides by this means. 



The researches of Oura^^^ and of Kuwada et alii^^* on the effect of 

 treating chromosomes with dilute alkalis has shown that some con- 

 tracted plant chromosomes can be partly unravelled to exhibit a clear 

 spiral structure. Shigenaga^^'' has used neutral sodium chloride for the 

 same purpose. Both of these agents are solvents for DNA. 



100 



