MANIPULATION OF EGGS 107 



the coverglass and a piece of filter paper is held against the opposing 

 edge to absorb the fluid from that side. A convenient fixative is a 

 mixture of 5 ml glacial acetic acid and 95 ml absolute ethyl alcohol; 

 nuclear structures can then be satisfactorily stained with a o-i per 

 cent aqueous solution of toluidine blue. After such treatment, the 

 edges of the coverglass can be sealed with paraffin or beeswax so as 

 to make a semi-permanent preparation. Some authors prefer to fix 

 and stain the eggs with the use of a single solution, and good results 

 have been obtained with aceto-carmine (0-5 per cent carmine dis- 

 solved in 45 per cent acetic acid) (Chang, 1952a; Spalding, Berry 

 and Momt, 1955; Berry and Savcry, 1958; Hancock, 1958). 



The optical equipment most generally preferred for the high- 

 power study of living eggs is the phase-contrast microscope fitted 

 with negative contrast objectives. Illumination for viewing is best 

 obtained from a very bright point source, the light passing through 

 a monochromatic green filter; for photomicrography, the filter 

 should be appropriate to the type of emulsion used. An alternative 

 optical system is the anoptral phase contrast, which is said to have 

 some advantages, notably the avoidance of flare around highly 

 refractile structures (Wilska, 1954). The interference microscope, 

 invaluable for the study of tissue-culture cells and the like — since it 

 permits the determination of dry-matter content and presents a very 

 satisfactory colour-contrast picture at low magnifications (Hale, 

 1958) — is not appropriate for detailed observations on eggs owing to 

 their large size and manifold inclusions, and because resolution is 

 poor at high power. Another recent development is fluorescence 

 microscopy, in which eggs treated with vital fluorochromes such 

 as acridine orange are subjected to ultra-violet radiation of relatively 

 long wavelength and examined with a conventional bright-field 

 microscope fitted with a dark-ground condenser. With acridine- 

 orange staining, information can be obtained on the distribution in 

 living eggs of dna which gives a bright green fluorescence; the 

 striking red fluorescence which granular bodies display seems likely 

 to be due to mononucleotides (Austin and Bishop, 1959a). Finally, 

 there is the ultra-violet microscope, the use of which offers two 

 advantages: with radiations of the shorter wavelengths available, 

 higher degrees of resolution can be obtained than with light micro- 

 scopy, and the distribution of substances having sharp absorption 

 maxima, such as the nucleic acids, can be studied. The characteristic 

 strong absorption of nucleic acids at a wavelength of 2,600 A is 



