CELLS OF THE COLON BACILLUS 93 



Sufficient information has now been presented to warrant some 

 speculation regarding the significance of these variations in the size 

 and form of the cells of bacteria during the period of active growth. 

 While at first glance the simple explanation of Clark and Ruehl 

 that these variations are merely due to the growth of the cells pre- 

 paratory to cell division would seem sufficient, it should be clear 

 by now that such an explanation is quite inadequate. I have previ- 

 ously mentioned my belief that these changes in size, considered 

 together with certain other facts, are an indication that during the 

 period of active growth we have, with bacteria, to deal with a special 

 cell type characteristic of this growth phase, different in internal 

 constitution as well as in external form from the resting cells with 

 which we are mostly familiar. These young cells are therefore 

 in a sense analogous to the embryonic cells of a growing multicellular 

 organism. Now if we look upon them as having actually the signifi- 

 cance of such embryonic cells, there is opened up an attractive field 

 of speculation from the standpoint of phylogeny, for such embryonic 

 cells should tend to revert to an ancestral type. The fact that with 

 most species of bacteria these embryonic cells are long and slender, 

 tending towards a filamentous form, suggests the possibility that 

 these are attempting to form mycelium ; that the bacteria are actually 

 descendents of higher fungi. Such an hypothesis is, however, pure 

 speculation, hardly warranted by the data at hand. 



More profit would seem to accrue from a consideration of the 

 data from a physical standpoint. Since the size of the cells con- 

 tinually increases as long as the growth rate is accelerating, the 

 synthesis of protoplasm during this phase proceeds more rapidly 

 than does cell division; and conversely, since the cells become in- 

 creasingly smaller when the growth rate becomes slower, cell division 

 must in this period proceed more rapidly than does the synthesis 

 of protoplasm. Since increase in size is accompanied by increased 

 relative slenderness, i.e., by a greater tendency towards the cylindrical 

 or filamentous form, and the subsequent decrease is accompanied 

 by a greater and greater tendency towards the spherical form, it 

 follows that during the period of accelerating growth there occurs 

 an increased amount of surface in proportion to the volume of 

 protoplasm for each cell, and that during the phase of negative 



