The first 40 years of this century witnessed bacteriologists involved in a debate which was fought with an intensity not seen since the arguments over spontaneous generation conducted during the last quarter of the 19th century. This now long-forgotten controversy concerned the question of whether or not bacteria exhibit extreme pleomorphism and go through complex life cycles. The term pleomorphismsvzs. used to refer to the supposed ability of bacteria to change shape dramatically, or to exist in a number of extreme morphological forms. Thus it was believed that bacteria could change from a single coccoid to complex filamentous forms and vice versa. In addition, rather than reproducing by single division, bacteria were thought to undergo complex life cycles involving single cells, spore, filaments, and ultra-filterable forms.The debate split microbiologists into two opposing schools: the monomorphists and the pleomorphists. The monomorphists finally triumphed, but as we shall see, even today reports continue to appear apparently showing that bacteria exhibit extreme morphological variations and undergo complex life cycles.Nearly all modern microbiologists belong to the monomorphic school; that is, they accept that, apart from minor variation, each bacterial cell is derived from a previously existing cell of practically the same size and shape. Cocci generally beget cocci, and rods give rise to rods. The monomorphist view, stressed by Virchow, Cohn, and Koch, is that by binary fission most bacteria divide transversely to produce two new cells which eventually achieve the same size and morphology of the original. In the same way, a single spore germinates to give rise to a vegetative cell essentially the same as the cell from which the spore originated. Exceptions to this rule are accepted in certain so-called higher bacteria, including some actinomycetes. Simple bacteria, on the other hand, are generally regarded as show-