Protoplasm, the Cell, and the Organism - 21 



-, 



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Fig. 2-7. Two nerve cells in the spinal cord. Each 

 nerve fiber is an elongate extension from a particular 

 nerve cell. Some fibers reach parts of the body several 

 feet distant from the main part of the cell. The nuclei 

 do not show clearly, due to overstaining. (Copyright, 

 General Biological Supply House, Inc.) 



surround itself with a rigid or semirigid non- 

 living coating, which holds the protoplasm in 



plasm (p. 94) or by pressure from surround- 

 ing cells. 



Intracellular Structure. The various types 

 of light microscopes available today (Fig. 

 2-10) enable one to observe the nucleus and 

 other major intracellular structures in some 

 detail; but the finer features of such struc- 

 tures cannot be resolved by any of these light- 

 focusing instruments. Cytologists, who spe- 

 cialize in the study of cell structure, were 

 greatly handicapped until quite recently, 

 when the development of the electron micro- 

 scope and associated techniques began to 

 reach fruition. 



The limit of resolution of a light micro- 

 scope is reached when the diameter of any 

 spherical or cylindrical structure falls slightly 

 below 0.1 micron (Fig. 2-6). Specifically, reso- 

 lution fails when the structural dimension 

 diminishes to about half the wavelength of 

 the type of radiation being utilized. Thus 

 ultraviolet light, although not visible to the 

 eye directly, permits a photographic resolu- 

 tion of fine structures down to about 0.06 

 micron. But the electron beam extends the 

 resolution even further, down to about 0.0001 

 micron (1.0 millimicron). Today, therefore, 

 the fine structure, or ultrastructure, of the 

 protoplasmic parts can be examined, thus 



a definite mold; or as with a few cells, it may affording us a deeper understanding of the 

 construct a delicate internal skeleton that vital activities of living cells, 

 supports the protoplasm. Also, cells may be There are many pitfalls in cytology, how- 

 stabilized in shape by a gelling of the proto- ever. Usually it is necessary to kill and fix the 



FREE 

 SURFACE 



SQUAMOUS 



COLUMNAR 



CUBOIDAL 



Fig. 2-8. Note that epithelial cells display virtually no intercellular material and 

 that they cover the surface very completely. The three types shown here are: the 

 tall columnar epithelium; the cuboidal epithelium; and the scalelike or squamous 

 epithelium. 



