STUDY or PELOMTXA PALUSTEIS. 377 



Having thus established satisfactorilj^ that the rods were 

 •capable of independent motion of a bacterial nature, attention 

 •was next directed to watching for their division. 



Division. — It was fortunately possible, bj the same process of 

 continuous observation, to establish the fact of division of a rod. 

 ¥'or this purpose, however, periods of 3 hours were not enough ; 

 the observation required to be absolutely unbroken for the whole 

 day. My thanks are here due to my husband, who rendered 

 continuous observation possible by taking my place at the 

 microscope when short absences were necessary for meals. 



The 28th and 29th of December were spent in this manner, 

 and during these two days I observed division several times, a 

 single rod of course being kept under observation in each case. 

 Division took place in the following manner : a rod consisted of 

 two equal joints A B ; A formed a third joint by the appearance 

 of a new constriction dividing it exactly at the middle. At this 

 stage the rod presented the appearance of being divided into one 

 long joint and two short ones. Sometimes the newly-made 

 joints rapidly grew to equal length with joint B before further 

 division, and this accounted for the rods with an odd number of 

 joints (three or five) so commonly found. Sometimes, however, 

 joint B followed suit almost immediately with division, resulting 

 in the formation of a rod with four joints of equal length. In 

 this case history repeated itself, viz., either one of the terminal 

 joints divided again (5 joints), or both terminals did so, producing 

 a six-jointed rod. Now, and now only, so far as I was able to 

 observe, did actual separation take place, a4-jointed rod breaking 

 in half, or a 6-jointed rod breaking off two of the terminal joints 

 ill one piece. 



This appears to be absolutely characteristic of the organism 

 under normal conditions, and accounts for the fact that free 

 single-cell rods are, as far as my experience goes, never found ; 

 hence the necessity of starting from a two-jointed individual, 

 which at first sight seems not to be beginning at the beginning. 

 This division from a double unit surprised me exceedingly, and I 

 am not aware if such a mode of division has any parallel among 

 known life-histories ; however, the fact remains that subsequent 

 investigations only served to confirm the observation, and indeed 

 it afterwards formed an important means of identification. 



Once an 8-jointed rod was seen to break off four joints, but 

 in no case was the unit set free a single one. Division was by 



