MICRURGY 53 



best suit different types of work; a heavier needle (Fig. 39C) will 

 better penetrate the cellulose wall of a cell than a slender one. 

 The needle tip must now be bent, preferably slightly less than 

 a full right angle, so that two points can be brought into con- 

 tact (Fig. 36). The nature of the bend must also suit the work 

 at hand. For cutting, a greater slope is needed; and for entering 

 plant cells horizontally, a double bend with a gradually rising 

 tip is necessary (Fig. 40). Peterfi has added several other 

 dissecting tools, such as a micropincer, microknife, and micro- 

 spatula made from the scale of a butterfly wing (Fig. 39D). 



With moist chamber and needles made, we can proceed to 

 dissect. The material, which must be thin and reasonably 



IT 

 n 

 Fig. 40. — Plant tissue mounted in a hanging drop ready for dissection; n, needle; 

 t, tissue; w, water film; c, glass cover slip. 



transparent, may be an amoeba, an egg, the epidermis of a leaf, 

 the Plasmodium of a slime mold, etc. It is placed in a film 

 of water on a glass slip. The latter is then inverted and placed 

 upon the moist chamber. The two needle tips are brought 

 into position under the living cell and gradually raised until 

 the points are in the cell. The needles are then moved in any 

 direction by the thumbscrews of the instrument. 



J. Comandon and P. de Fonbrune have used another method 

 to prevent evaporation of the water in which the material is 

 suspended. They have injected a droplet of blood, containing 

 the cells in which they are interested, between a drop of oil 

 and the surface of the cover slip. The outer covering of oil 

 prevents evaporation and yet permits the entrance and manipula- 

 tion of microneedles and pipettes. The method eliminates 

 the necessity of a moist chamber, thus permitting dissection 

 with the droplet exposed without danger of drying out. 



Microinjection. — The effect of salts on protoplasm is occupy- 

 ing the attention of cell physiologists at present more than is 

 the anatomy of the cell. Tissues can be immersed in salts, 



