cytology: the study of the cell 133 



particular structures and leave others unstained. Such dyes, however, will 

 function as a rule only after the cell has been killed. The cytologist is com- 

 pelled, therefore, to kill the cell first, then stain it. This necessity introduces 

 into the work an element of uncertainty, for one is confronted with the possi- 

 bility that the killing process has altered the fine structure of the cell or has 

 rendered parts of the cell soluble in the reagents that must be used in the 

 staining and mounting process. To be able to get away from the necessity 

 of killing the cell, to find a way of seeing the structures in living protoplasm, 

 has been the dream of the cytologist from the beginning. He now has two 

 new kinds of microscopes, the phase-contrast microscope and the interfer- 

 ence microscope, which utilize certain optical principles hitherto unused in 

 microscopy to make many of the constituent parts of the cell stand out from 

 their surroundings when the protoplasm is still living, so that they can be 

 studied in respect to both their structure and behavior. 



Perhaps the most spectacular mechanical development which in recent 

 years has become available to the cytologist is the electron microscope. This 

 instrument is capable of providing sharply resolved images at magnifications 

 10 times that of the light microscope. Although there are certain drawbacks 

 to its use, the greatly increased magnification which it affords has brought 

 it into great prominence as a cytological tool. The drawbacks to its use, 

 beside the great cost of the instrument, are the fact that it must be used 

 primarily as a photographic instrument; the fact that the material studied 

 must be very thin, since electrons will not pass through thick layers of 

 matter; and the requirement that material must be studied in a vacuum, 

 since electrons will not penetrate air. These difficulties are gradually being 

 overcome to a considerable degree. Microtomes have been invented which 

 will cut amazingly thin sections, and good methods are being developed 

 whereby the object of study can be evacuated and consequently desiccated, 

 with much less distortion of the fine structure than was the case at first. 

 The use of the electron microscope as a photographic instrument is in one 

 respect an asset; for it is possible to take pictures at magnifications of 15,000 

 diameters or more on fine-grained film, from which enlargements can be made 

 which retain sharp definition up to magnifications of 50,000 or more. 



The electron microscope is making as drastic a revolution in cytological 

 knowledge as did the light microscope in the 1870's and 1880's. It is reveal- 

 ing an amazing intricacy of structure in the cell beyond anything dreamed 

 of a few years ago. Resolving powers as low as 8 Angstroms are being 

 achieved; i.e., it is possible to reveal particles as small as one thirty-millionth 

 of an inch in diameter. The discovery of structures and types of organization 

 within the cell hitherto unsuspected is a long step in the direction of under- 

 standing the nature of the living machine and how it functions. 



At its inception, cytology was a distinct science which showed relatively 

 little relationship to other sciences. Its objective was to know the cell, its 



