256 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1954 



histology and embryology from the point of view of the new detail 

 that is to be seen. Artifacts formed during specimen preparation 

 obviously are more troublesome the finer the structures being studied, 

 and new methods must be developed before we can utilize to the full 

 the potentialities of the electron microscope. Nevertheless, the prog- 

 ress already made has revealed far more than we can yet interpret 

 within sectioned tissues. 



Imiumerable questions can be answered from the examination of 

 healthy cells. If they are from plants we can, for example, follow 

 the growth of tlieir walls and we can examine the line structure of 

 their chloropiasts, of their starch grains, and of the many other objects 

 to be seen in their cytoplasm. If they are of animal origin, we can 

 inquire into the elaborate mitochondrial and other structures that are 

 particularly striking in cells of special function such as those in the 

 liver or kidney, in the eye or the nervous system. With both plant 

 and animal cells we can approach from this liner level of organiza- 

 tion the mechanism of cell division (pi. 2, tig. 1) . 



These studies of components of the normal cell and of how they 

 develop supply the background requisite for investigathig the ab- 

 normalities of cell life that are responsible for man}'^ diseases of living 

 matter. Virus diseases are among the first of these to which we 

 naturally turn, both because the viruses tliat cause them are too small 

 to have been visible before and because, whatever their nature, these 

 pathogenic agents stand on the borderline between the animate and 

 inanimate and thus offer a unique opportunity of learning more about 

 the essence of the living state. 



My laboratory in the United States at the National Institutes of 

 Health has been particularly concerned over the past few years with 

 this study of virus growth, and we have learned much about how 

 several viruses develop within the cells that are their hosts. There 

 appears to be no uniform pattern for this development, and one can- 

 not briefly describe the various types of virus growth we have seen. 

 Nevertheless, I would like in conclusion to indicate what we have 

 found about one familiar virus — that of influenza. From an examina- 

 tion of purified suspensions, we know that its infectious particles are 

 either tiny spheres or filaments of the same diameter. In sections 

 through diseased tissues we fuid these virus particles not within the 

 infected cells themselves, as many of us would have expected, but 

 clustered around their peripheries and developing from them as fila- 

 ments that break off and segment into spheres (pi. 2, fig. 2). Such 

 observations with the electron microscope demonstrate that the ele- 

 mentary infectious units of this virus have not proliferated after the 

 fashion of minute micro-organisms but instead are bits of cytoplasm 

 of the diseased cells. 



