CHAPTER II 



THE GENERAL PRINCIPLES OF THE METHOD THE 



APPARATUS REQUIRED THE SPECIAL PHOTOMICRO- 



GRAPHIC APPARATUS THE REVOLVING APPARATUS. 



THIS method by which cells are observed in vitro is 

 very simple. They are placed on a film of agar jelly, 

 which holds in solution any material with which we 

 may w r ish to experiment. To prepare the film, a drop 

 of molten jelly is poured on to a slide, which is then 

 laid on a level surface until the jelly sets firmly. A 

 drop of the citrate solution in which, say, blood-cells 

 are suspended is then placed upon a cover-glass, which 

 is inverted and allowed to fall flat on the film. It 

 might be thought that the weight of the cover-glass 

 would be sufficient to kill the cells; but they sink into 

 the jelly to' some extent, and so become protected. 

 Before this happens, however, they spread out centri- 

 fugally from the centre to the periphery of the cover- 

 glass, and if a drop of blood be examined in this way 

 on stain-containing jelly they may be seen by the 

 naked eye rushing in every direction towards the edges 

 of the cover-glass. When this movement has ceased, 

 if the slide is held up between the observer and the 



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