580 



REPRODUCTION. 



observed the occurrence of sexual generation in 4t different genera and 

 66 different species. 



Thus the infusoria are in turn excluded from the field of spon- 

 taneous generation. But, on the other hand, a considerable group of 

 organisms, formerly referred to this class, are now known to be of a 

 different character. These are the forms included under the general 



FIG. 156. 



STYLONYCHIA MYTILUS ; a fresh- water infusorium. 1. Unimpregnated. 2. Impregnated, and con- 

 taining mature eggs and two embryos. 3. Showing the form of the embryo. Magnified 375 

 diameters. (Stein.) 



term of Bacteria, and comprising the varieties of bacterium, vibrio, 

 spirillum, and micrococcus. They are of a vegetable nature, notwith- 

 standing their frequent exhibition of active movement ; and they consist 

 of cells, which multiply, often with great rapidity, by repeated subdi- 

 vision. Whether they are also reproduced by germs, has not been deter- 

 mined ; but their minute size and imperfect classification have thus far 

 proved obstacles to the complete study of their physiological characters. 

 The representative of this group is the species known as Bacterium 

 termo, already described (page 78), in connection with the phenomena 

 of putrefaction. It consists of rod-like cells, averaging 3 mmm. in 

 length by 0.6 mmm. in thickness, sometimes single, often double, two 



