408 AN INTRODUCTION TO MODERN GENETICS 



batches of stain and should be tried out first. For best results, the larvae 

 should be overfed with yeast and grown at a low temperature. 



Preparations made as above can be kept for some days if the cover- 

 glass is sealed with vaseline. Permanent preparations can be made by 

 the following method. The smears are made on slides which have been 

 thinly wiped with albumen solution, which is then allowed to dry 

 completely. After staining and pressing out the glands, place the slide 

 in a closed stain jar with enough 95 per cent alcohol just to cover one 

 or two millimetres of the lower part of the cover-glass. In about half 

 an hour, the alcohol vapour will have permeated the liquid film between 

 cover-glass and shde. Transfer to a full jar of 95 per cent alcohol, and 

 after some time remove the cover-glass carefully, when the stained 

 glands should remain attached to the slide. Mount in Euparal. 



Similar methods can be used for the demonstration of mitotic and 

 meoitic chromosomes. For the latter, pollen mother cells of liliaceous 

 plants or spermatocytes of Orthoptera are recommended. 



b. Collecting Eggs 



Valuable experiments on competition and selective advantages can 

 be made by setting up overcrowded cultures of mixtures of two pheno- 

 types, and determining the ratio of the different types which eventually 

 emerge. This can be done in a somewhat imprecise way by varying 

 the number of parents or of ±e time given for egg laying, but more 

 satisfactorily by planting out definite numbers of eggs. Eggs can be 

 most easily collected in either of the following ways : Put the flies in 

 an empty bottle or vial with a microscope slide which is thinly (i nmi.) 

 smeared with heavily yeasted food, or close the bottle with a cardboard 

 cap carrying a smear of food and stand the bottle upside down, so 

 that the food is below the flies. The eggs can be more easily seen if the 

 food is darkened by admixture of carbon. Eggs are only about • 2 mm. 

 long and must be handled under a binocular microscope. The stage 

 of development at the time of laying is variable, depending on how 

 long they have been retained in the oviduct and uterus of the female. 



