PREPARATION OF THE SPECIMEN 17 



its course is arrested because it has adhered to the 

 jelly, or between the jelly and the cover-glass. Some- 

 times the rest may be only momentary, when the cell 

 may be seen to revolve on its own axis for a few mo- 

 ments, and then pass on again in the slowing stream. 

 Leucocyte after leucocyte afterwards becomes arrested 

 in this way; they apparently stop first because they 

 are larger and more "sticky." Then the red cells 

 gradually stop, until at last the field is dotted with 

 living blood-corpuscles, which may happen to become 

 arranged in groups or rest singly side by side. 



The specimen may now be moved about by means 

 of the mechanical stage, when it will be seen that all 

 the cells in the film of blood under the cover-glass 

 have become arranged in a manner very suitable for 

 examination. The frontispiece of this book is a 

 photomicrograph of a typical field presented by this 

 method. 



The living cells all come to rest in a short time, and 

 each one has its own share of jelly-surface, from which 

 it has no alternative but to absorb any substances 

 which have been previously dissolved in the jelly. 

 Having focused a field, therefore, which contains an 

 example of the cell w r ith which one wishes to experi- 

 ment, it is only necessary to wait until that cell has 

 sufficiently absorbed the contents of the jelly for it to 

 respond to the agent which has been dissolved in it. 



"Artefacts" do not exist; the surface of the jelly 

 is the same all over. One has no control over the 

 attitude which a cell may adopt, no matter what 

 part of the jelly-surface it may come to rest upon, nor 



