106 THE MAMMALIAN EGG 



it there by maintaining slight negative pressure within the pipette. 

 Alternatively, larger-bore pipettes may be preferred for the larger 

 objects. 



After the egg has been placed on the slide, it is covered by a 

 coverglass to the edges of which a little vaseline has been applied. 

 The purpose of the vaseline is to prevent the coverglass from being 

 drawn down close to the slide by the surface tension of the fluid — 

 which would be very likely to crush the egg — and to permit some 

 control of the compression applied to the egg. Spaces should be 

 left in the vaseline edging to allow the escape of air and medium. 

 It is recommended that the volume of medium deposited with the 

 egg on the slide should be as small as practicable — if the volume is 

 too large the fluid may run to the edge of the coverglass, carrying 

 the egg with it. This consideration is especially important with 

 denuded eggs and when several have been placed on the one slide; 

 in studies with a high-powered microscope, it is specially convenient 

 to have all the eggs close together, thus avoiding the need to hunt 

 for each one over a wide area. 



Once the coverglass is in position, and contact has been made 

 with the fluid droplet, pressure is applied with the fingers to opposite 

 edges of the coverglass while progress is watched through a dissect- 

 ing microscope. The coverglass is depressed until it just makes 

 contact with the surface of the egg or with cells closely investing it. 

 The slide is then transferred to the stage of a high-powered micro- 

 scope and compression continued in the same way while the results 

 are observed at low magnification (16-mm objective). Within 

 limits, the more the egg is flattened the clearer will the internal 

 details be at high magnification (2-mm objective), but some experi- 

 ence is needed to know just how much an egg can be compressed; 

 excess pressure will either rupture the egg or cause it to degenerate 

 rapidly. When suitable flattening has been achieved, more medium 

 may be run under the coverglass to prevent the preparation from 

 drying out. If flattening has not been excessive, it is generally 

 possible to change the orientation of structures within the egg, and 

 so obtain optimal presentation of a selected detail, by gently sliding 

 the coverglass and so rolling the egg. Sometimes, however, the egg 

 becomes adherent to one of the glass surfaces and will not roll. 



Eggs set up on a slide in this way may be fixed and stained by 

 drawing the appropriate solutions under the coverglass: a drop of 

 the solution is deposited on the slide in contact with one edge of 



