Micro: 2 



300,000 diameters. A nematode one millimeter long, at this scale of 

 magnification, would appear a little over a sixth of a mile long. 



The limitations presently inherent in the use of these instmiments are 

 serious ones to contend with. " Biological materials must be on the order 

 of only 0.1-0.2 ji maximura thickness, if internal details are to be seen. 

 New procedures and equipment developments in microtomy and freeze-drying 

 now render this a much less difficult requirement to be met. 



The ultra thin slice of the specimen is placed within the tube of the 

 microscope for exposure to the beam of electrons. The interior of this 

 tube then has to be highly evacuated to permit steady and prolonged 

 operation and to avoid scattering of the electrons from the beam by 

 chance collisions with residual gas molecules. Thus, the specimens are 

 exposed to pressures not exceeding 10"^ mm. of mercury, at the most. 

 Such low pressures are attained in about four minutes. Living tissue 

 cannot survive this. There is said to be some prospect for eventual 

 viewing of biological specimens in the wet state. The beam of electrons 

 also is not without effect on the specimen. There is an electrical 

 charge involved, although this has only a minor effect on the specimen. 

 However, heat is conveyed to the specimen by the beam and this is a 

 serious problem. At moderate illumination intensities, specimen tem- 

 peratures of 150-200° C. are reached, although using special precautions, 

 the specimen temperatures can be held to 50-60° G. Chemical changes in 

 biological materials also occur as a result of the exposure to electrons. 



In summary then, nematode specimens can not be observed in the living 

 state by electron microscopy. They must be dehydrated and sectioned to 

 about 0.1 ^. Considerable skill would be required in preparing the 

 specimen mount for observation, particularly if cross-sections of the 

 typical eel-shaped nematodes are wanted. However, the unsurpassed 

 resolution and magnification of the electron microscope can be brought 

 to bear on special problems of nematode structure. This has been done 

 at Cornell on the study of the cyst wall of Heterodera rostochiensis . 

 Anyone in convenient range of access to the services of an electron 

 microscopist would surely find some worthwhile facts on such important 

 nematode stnictures as the cuticle and egg shell and, perhaps, nematode 

 relationships with viruses. 



X-Ray Microscopy 



There is now a new type of microscope which utilizes X-rays instead of a 

 light or electron beam. The greater penetrating power of X-rays reveal 

 structures which may be impossible with the light or electron rays. The 

 presently available X-ray instruments operate in the magnification range 

 of the standard bright field light microscopes and with resolving po\:er 

 said to be as good or better than the best optical microscopes, or on 

 about par with the ultraviolet microscope. The limitations at present 

 are only technical ones, and the X-ray microscope can be e.x}^ectod to 

 improve as it is not at its theoretical limits yet. 



