THE application of the electron microscope to the study of the structure of polyraorphonuclear leucocytes began 20 years ago. At that time quite good electron microscopes were available, but satisfactory methods for preparing sections of biological material for electron microvscopical examination had yet to be devised. It was, however, possible to examine small biological specimens if they could be dried dov^ Ti onto thin supporting films covering copper grids. This kind of technique provided (and continues to provide) important information about very small objects such as virus ijarticlcs, bacterial appendage. s, fibres and certain intracellular structures. Techniques of this kind are less informative when appHed to whole animal cells, which are inevitably much distorted by the drying process and moreover are so thick that in places they are completely opaque to the electron beam, while even in their inner parts the resolution obtained is disappointingly poor.

Using such procedures, polymorphonuclear leucocytes were examined in the electron microscope by Rebuck and Woods (1948) and by Bcssis and Bricka (1940, 1950). In the last mentioned study the authors not only examined whole colls but also granules from disrupted ncutrophil and eosinophil polymorphs, and succeeded in illustrating the shapes of the granules much more clearly than had ever been possible by light microscopy. Nevertheless, studies by such methods are now of historical interest only. The earliest electron micrographs of polymorphonuclear leucocytes as seen in thin sections were pubhshed by Braunsteiner et al.(1953), Kautz and De Marsh (1954, 1955), Watanabe (1954), Bernhard et al.(1955), Grey and Biesele (1955) and Pease (1955, 1956). Among these pioneer studies that of Kautz and De Marsh (1954) was noteworthy for the high quality of the electron micrographs, considering the date. The principal features of neutrophil, eosinophil and basophil polymorphs were clearly demonstrated, and although much detailed information has since been added, the initial conclusions of these authors have not been upset. Since then, electron microscopical studies mainly or incidentally concerned vnth the structure of the polymorphonuclear leucocyte bave been so numerous that it would be well-nigh impossible to provide a eomprehensive review ami inappropriate in the present communication to attempt more than to refer to illustrative examples of contributions to later lines of research.