THE IRON-ACETOCARMINE METHOD. 



161 



clears the specimens sufficiently. (Too much chloral hydrate 

 will cause shrinkage.) The pollen-grains are squeezed or partly 

 crushed in this liquid, and may be left to stain for a day or two. 



FIG. i. Hyacinth zn. 



In the case of pollen-mother-cells, local pressure, with a small 

 roll of paper, on the cover-glass, often spreads out the cytoplasm 

 on the cover-glass or slide, with the chromosomes uninjured. 

 If required, these can be preserved in balsam, by replacing the 

 liquid by graduated mixtures of 45 per cent, acetic and alcohol. 

 Fig. i is a balsam preparation of the reduction metaphase in a 

 hyacinth, prepared in this way. 



* 



* 



* 



FIG. 2. Datura zn i. 



The cytoplasm becomes quite clear in the acetic acid, and with 

 an appropriate green screen on the microscope, the bluish red 

 chromosomes stand out jet black. (Wratten green films Nos. 

 66 and 56, are most useful on a binocular, and No. 58 on a 

 monocular, with a good tungsten electric light.) Fig. 2, a pollen- 

 mother-cell of Datura in the second metaphase, with 23 chromo- 

 somes, in iron-acetocarmine, shows the sharpness of the images. 



Pressure is usually best applied from 2 to 7 days after mounting, 

 but the time varies for different pollen-mother-cells. Well pre- 

 pared slides will often keep for some months. 



