I. The existence of differences and similarities in the sur- 4.
face features not only of different organisms or groups but also within given species has been demonstrated by a variety of techniques.
2. The different reactions of various organisms to the Gram stain might well be taken as one piece of evidence, the use of the electron microscop and associated preparative techniques (including direct shadows, thin sections, negative stains, and freeze-etches) provided clear indications and details for such variations.
3. Many of these preparative procedures when used alone, however, do not provide adequate views, and thereby sufficient information, to permit the desired comprehension of the three dimensional nature of the cell surface.
Direct shadows or the preparation of carbon replicas by the use of metals of fine grain (e.g., platinum-carbon) afford the opportunity to determine the characteristics of cell surfaces and to provide cleares understanding of the nature of such surfaces than is possible with negative staining. It is thus possible to observe, for example, not only gross morphological features but also globular or meshlike surfaces with these techniques; clearly, however, only exposed surfaces can be viewed. When spores of B. polymyxa and B. fastidiosus are compared by means of the carbon replica the utility of the freeze-etching approach, particularly for analysis of surface structures and their component layers, becomes readily appeant.