680 



NATURE OF REPRODUCTION. 



Fig. 219. 



observed the occurrence of sexual generation in 47 different genera 

 and 66 different species. 



Thus the infusoria proper are in their turn excluded from the field 

 of spontaneous generation. But, on the other hand, a considerable 

 group of organisms, formerly referred to the class of infusoria, are now 

 known to be of a different character. These are the forms included 

 under the general term of Bacteria, and comprising the special varieties 

 of bacterium, vibrio, spirillum, and micrococcus. They are demon- 

 strated to be of a vegetable nature, notwithstanding their frequent 

 exhibition of rapid and continuous movement; and they consist of cells, 

 which multiply, often in great abundance, by a process of repeated sub- 

 division. Whether they are also reproduced by means of spores or 

 germs, has not been determined ; but their minute size and their imper- 

 fect classification have thus far proved ^obstacles to the complete study 

 of their physiological characters. 



The representative of this group may be considered to be the species 

 known as Bacterium termo, already described (page 83), in connection 



with the phenomena of pu- 

 trefaction. It consists of 

 elongated or rod-like cells, 

 averaging 3 mmm. in length 

 by 0.6 mmm. in thickness, 

 sometimes single, often 

 double, two of them being 

 attached, more or less firmly, 

 end to end. The latter ap- 

 pearance is due to the pro- 

 gressive multiplication of 

 the cells, which takes place 

 by a transverse division at 

 the middle of their length. 

 The two new cells thus pro- 

 duced remain for a time in 

 connection with each other, 

 and afterward separate, to 

 repeat the process indepen- 

 dently of each other. The final separation of two cells may often be 

 seen to occur under the microscope. The bacterium cells, during a 

 considerable part of their existence, are in rapid vibratory and progres- 

 sive movement. The vibrations take place in a circular manner, about 

 some point situated either at or near one of the extremities ; so that the 

 rest of the cell performs a conical movement around this point, present- 

 ing, on superficial examination, the appearance of a lateral oscillation. 

 The mechanism by which this vibration is accomplished is unknown; 

 but it is no doubt analogous to the slower spiral undulations of the 

 Oscillatorise, among fresh-water algae ; and its effect is to propel the 



Cells of BACTERIUM TERMO; from a putrefying 

 infusion. 



