THE NATURE OF REPRODUCTION. 



581 



FIG. 157. 



CELLS OF BACTERIUM TBKMO ; from a putrefying 

 infusion. 



of them being attached, end to end. The latter appearance is due to 

 their multiplication by transverse division, which takes place at the 

 middle of their length. The two new cells thus produced remain for a 

 time in connection with each other, and afterward separate, to repeat 

 the process independently. Dur- 

 ing a considerable part of their 

 existence, the cells are in rapid 

 vibratory and progressive move- 

 ment. The vibrations take place 

 in a circular manner, about a 

 point situated at or near one of 

 the extremities ; so that the rest 

 of the cell performs a conical 

 movement around this point, 

 presenting, on superficial exami- 

 nation, the appearance of lateral 

 oscillation. The mechanism by 

 which this movement is accom- 

 plished is unknown ; but it is no 

 doubt analogous to the slower 

 spiral undulations of the Oscil- 

 latoriae, among fresh- water algae; 

 and its effect is to propel the 



bacterium cells, often with extreme velocity, through the fluid in which 

 they are immersed. 



Of late years, the investigations in regard to spontaneous generation 

 have been mainly confined to bacteria and their allies, since they now 

 form the only group of organisms in which reproduction by generation 

 has not been established. Even for them, the rapid multiplication by 

 cell division, which takes place under favorable conditions, indicates 

 their usual mode of increase ; but in order to establish an entire simi- 

 larity between them and other living organisms, they must also be 

 shown to reproduce by spores or germs, which thus far has not been 

 done. The experiments with boiled infusions in sealed flasks have led 

 to results which are not interpreted in the same manner by all writers ; 

 but it is evident that for bacteria, as well as for other organic forms, 

 the application of heat exerts in various degrees a preventive action on 

 their subsequent appearance. 



The experiments of Wyman* on this subject were performed with 

 animal and vegetable infusions, which, after being enclosed in sealed 

 flasks, with atmospheric air, were immersed in boiling water for periods 

 varying from thirty minutes to five hours, and afterward kept under 

 observation at temperatures favorable for the development of bacteria. 

 The result showed that the appearance of these organisms was always 

 delayed by the application of a boiling heat, and that this delay was 



* American Journal of Science and Arts. New Haven, vol. xliv., September, 1867. 



