18 THE GENERAL PRINCIPLES OF THE METHOD 



over the other cells which form its immediate sur- 

 roundings. The cells are always placed on the jelly 

 in identically the same way as has just been described, 

 and therefore the only way in which one can in- 

 tentionally affect the individual cells is either by 

 deliberately (1) mixing some other substance with the 

 jelly before it is set on the slide, or (2) by keeping the 

 slide at various temperatures. It sometimes happens 

 that unintentionally the cells may become distorted 

 by the presence in their neighbourhood of some for- 

 eign substance which has been accidentally mixed with 

 them in the citrate solution in which they have been 

 suspended prior to being placed upon the film; but 

 such a foreign body may easily be recognized. 



The apparatus required for these researches is not 

 very elaborate. Many of the earlier experiments were 

 made in a cabin in a battleship, where there is not 

 much room for scientific apparatus, but we simply 

 enumerate them here for the benefit of those who may 

 desire to commence the study of in-vitro methods for 

 the first time. They consist of: 



1. Microscope slides. 



2. Cover-glasses. These should be very thin and 

 | of an inch in diameter. A few larger ones, say f of 

 an inch, may occasionally be needed. A silk hand- 

 kerchief is required to polish the cover-glasses, which 

 should be very clean and kept in alcohol. 



3. Capillary glass tubes. These are constantly in 

 use, and it is well to begin with a stock of 100 of them. 

 They should be about 4 inches long, having an internal 



