CELLS IN DIVISION 



the nineteen-twenties when workers such as Strangeways and Belar 

 showed that hving preparations could still yield fresh information 

 about the behaviour of cells in division. Belar's study of the classical 

 staminal hair cell of Tradescantia (Belar') led to a series of further 

 investigations on the same material by the Japanese school of cytolo- 

 gists, for with such refractile cells, the bright-field microscope is ade- 

 quate, as Belar's superb photomicrographs clearly demonstrate 

 (Plate V). In the study of the extremely thin cells in the outgrowth of 

 living tissue cultures, its limitations in contrast and resolution of un- 

 stained objects are more severe, though by no means prohibitive, as is 

 clearly shown by the researches of the Lewises (e.g. Lewis (W. H,)^ ^). 



It is now well known that thanks to the phase-contrast principle of 

 Zernicke these particular difficulties in microscopy are largely over- 

 come, and that the resolution of thin transparent objects can be equal 

 to, or even superior to that of stained preparations (Zernicke,^'' 

 Taylor,^^ Richards,^^ Hughes^^). Phase-contrast microscopy can be 

 very appropriately combined with serial photographic recording on 

 cinematograph film. Cells in division were first followed by serial 

 photography as far back as 1913 by Comandon and Jolly; in the 

 nineteen-twenties, Canti^* began his well known series of films on tissue 

 cultures. The first phase-contrast film was made in the early nineteen- 

 forties by Michel^ ^ on the meiotic divisions of an Orthopteran 

 spermatocyte, Michel's demonstration of the possibilities of phase 

 microscopy has led to a remarkable impetus in the study of the living 

 preparations of biological material. Studies on mitosis in tissue cultures, 

 both under normal and experimental conditions have been subse- 

 quently made by Hughes^^ ^^ and on malignant cells by Gey. 



The limiting factors in the wider investigation of further types of liv- 

 ing cell in division are both optical and biological. Some cells seem to 

 be so nearly optically homogeneous that isotropic retardation methods 

 such as phase contrast are inapplicable. For the rest, in which structural 

 details can be seen in life, suitable methods of cultivation are needed 

 to permit observations of healthy cells over a sufficient period of time. 

 By some means, a thin and preferably single layer of cells must be pre- 

 pared under conditions in which their normal activity will be fully 

 maintained. The tissue culture method is almost ideal for material 

 which can be grown in this way, provided that a rapid rate of cell 

 division can be maintained in the cultures; at present, this applies only 

 to a number of vertebrate tissues. The use of liquid paraffin for mount- 

 ing living material is surprisingly successful for Tradescantia staminal 

 hairs, and probably would be for other plant tissues. According to 

 Telezynski^^ oxygen dissolves more readily in paraffin than in water. 

 It was used by Michel for the spermatocytes oiPsophus, but seems less 

 effective for those of other Orthoptera. Further exploration of possible 



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