44 MORPHOLOGICAL CHANGES DURING GROWTH 



ting the projected image of the bacterium divided by the length squared, Henrici' has 

 obtained an index of the variation in form. The coefificient of variation in length of 

 cells, as well as in the area-length index, is increased during the period of increased 

 size. 



The various members of the Spirillaceae, because of their marked pleomorphism, 

 offer especially interesting opportunities for the use of this method. The three species 

 studied follow the usual series of changes in size. During rapid growth, the new cells 

 are long and plump and relatively straight save that where several organisms have 

 remained attached they show definite spirals. As reproduction becomes slower, the 

 individuals become more slender, more curved, and distinctly granular in staining. 

 During the period of decline unusual forms are observed — various bulging or budding 

 organisms and coccoid bodies both large and small. Henrici points out that "these 

 latter types are the forms which Lohnis and others have described as extraordinary 

 reproductive cells but that the trend of the growth curve would indicate that this is 

 not the case." 



The three members of the Mycobacteria studied need no especial comment save 

 to point out that the "senile" forms are distinctly granular with many coccoid bodies 

 present. 



Even the cocci pass through similar cyclical changes in morphology, although the 

 proportional difference between the diameter of the young and the senescent forms is 

 less than with most of the rods and spirilla. The four- to six-hour cultures contain 

 many deeply staining cocci approximately twice the diameter of those usually seen. 

 In studying B. megatherium, Henrici' has pointed out a number of factors which 

 affect the rate of progression through the period of maximum reproduction and con- 

 sequently the onset of the phase of decline. If the volume and constituents of the media 

 are constant and if varying numbers of the bacilli are inoculated, the fewer the cells 

 introduced the longer is the period of maximum reproduction and the greater is the 

 maximum size of the organisms. With the same seedings, if the nutrient ingredients 

 of the media are varied, then the richer the media the longer is the period of logarith- 

 mic growth and the greater is the maximum size attained. When transplantations 

 are made during the period of increasing size, the organisms in the subculture con- 

 tinue to increase in size, progressing even beyond the maximum reached by the parent- 

 culture. Transplantations made immediately after the parent-culture has returned 

 to the original size show no evidence of lag; the subcultures increase rapidly in size 

 again. After two or more hours in the stationary phase, however, subcultures show 

 an appreciable lag and do not progress beyond the curve of the parent-culture. Al- 

 though other organisms have not been studied with reference to these points, pre- 

 sumably they will follow the same laws. 



It is interesting to note that spore formation begins toward the end of the ac- 

 tive growth period, so that the factors which lengthen the phase of positive accelera- 

 tion in growth delay spore formation. 



It would appear, then, that even apparently simple bacteria growing under stand- 

 ardized conditions admirably fitted to suppress variation and to increase uniformity 

 pass through a series of cyclical changes which indicate a progression from youth 

 ' Henrici, A. T.: see various works cited previously. 



