BLOOD AND LYMPH. 89 



cently shown that in certain parts of the body, in 

 adult life, cells occur which in many respects re- 

 semble the nucleated red blood-cells of the embryo ; 

 such cells are found, for example, in the spleen, in 

 the red marrow of bones, etc. 



The most plausible theory in regard to the matter 

 is, that in certain parts of the body spleen, mar- 

 row, lymph-nodes, and liver white blood-cells are 

 produced, a part of which are changed into the red 

 blood-cells. The so-called nucleated red blood-cells 

 are supposed to be intermediate forms. It must be 

 remembered, however, that this view is not estab- 

 lished as yet, and many observers do not ascribe to 

 the so-called nucleated red blood-cells' the signifi- 

 cance upon which the advocates of this theory 

 insist. 



TECHNIQUE. 



Fresh Human Blood. This may be obtained by tying 

 a cord tightly around the finger to cause congestion, and 

 then pricking it sharply at the side of the nail with a 

 bright, clean sewing needle. A small drop is received on 

 a slide and covered at once, care being taken not to press 

 upon the cover-glass. The film of blood should be very 

 thin or the crowding of the cells will interfere with the 

 observation. As the plasma evaporates, the changes in 

 form due to shrinkage of the red blood-cells may be 

 observed near the edges of the preparation. Finally, a 

 drop of water should be allowed to run under the cover- 

 glass and its action observed, first, in causing the dis- 

 appearance of the crenulations on the shrunken cells and 



