INTRODUCTION 



a) the further physical and chemical characterization of the agent, b) its 

 genetic complexity (that is, whether it involves single characters, the full 

 hereditary material of the pneumococcus, or something intermediate), and c) 

 its cytological relationships to nuclear or extranuclear structures in these cells. 

 The current approaches to these problems are summarized in a number of 

 papers and reviews, (27, 91, 93, 130, 131). 



In addition to the volume of older literature which must be supposed 

 to carry some grain among the chaff, there are a number of more recent reports 

 of transformations in various organisms. These include E. coli (31 ) , Hemophilus 

 influenzae (22), Shigella paradysenteriae (133), Alkaligenes radiobacter (39), 

 and staphylococcus (38). Discussion of these and other transformations should 

 take into account complications which might arise from bacterial life cycles 

 more complex than usually regarded. 



This presentation has emphasized the contrasting features rather than 

 the similarities of the phenomena described as genetic recombination vs. trans- 

 formation. Both phenomena tend to the same genetic result: the elaboration 

 of the cells whose hereditary traits are derived from more than one parent. 

 When more is known of the morphological basis of "sexual" genetic recombina- 

 tion on one hand, and of the genetic properties of transformations, on the other, 

 a more profitable synthesis of these contrasting concepts may issue. 



The recrudescence of interest in bacterial cytology has been largely inde- 

 pendent of, though contemporary with the development of bacterial genetics 

 documented in this book (77). The geneticist notes at least two fields where 

 cytological information is indispensable to him: a) the form and behavior of 

 the bacterial nucleus, and b) the possible existence of complex "life cycles." 

 Reliable information on both these subjects is relatively meagre, but the 

 evidence of at least the existence of nuclei in bacterial cells is relatively convinc- 

 ing. Robinow's paper reprinted here (15) may be taken as one point of de- 

 parture for the more recent work on this subject (41, 74) . With the improvement 

 of techniques, we may look forward to rapid progress in the establishment of 

 the details of the nuclear cycle at cell division, segregation, etc. (99, 118, 92, 

 122, 47). 



The expression "life cycle" has come to carry many connotations in 

 bacteriology which hinder a careful discussion of the often conflicting observa- 

 tions of generations of bacteriologists. At least one type of cycle is indisputable, 

 the formation of highly resistant endospores by many bacteria. Whether 

 endosporogenesis has any genetic significance is controversial. At the least, 

 they may be supposed to represent the stage at which the presence of only one 

 nucleus per cell (in contrast to the two, four, or more, characteristic of most 

 rods) is the most likely (see 54), and on this basis may be particularly useful 

 for certain types of genetic experiments. More than one student of the mode of 

 nuclear segregation into the endospore has proposed a sequence of nuclear 

 fusion and meiotic reduction (i.e., autogamy). However, the cytological figures 

 are not easily interpreted, especially in view of the pitfalls of logical recon- 



