ANIMALS AND PLANTS. 173 



painstaking observers, who employed the highest attain- 

 able powers of the microscope and, relieving one another, 

 kept watch day and night over the same individual mo- 

 nads, have been enabled to trace out the whole history 

 of their Heteromita which they found in infusions of 

 the heads of fishes of the Cod tribe. 



Of the four monads described and figured by these 

 investigators, one, as I have said, very closely resembles 

 Heteromita lens in every particular, except that it has 

 a separately distinguishable central particle or " nucleus," 

 which is not certainly to be made out in Heteromita 

 lens; and that nothing is said by Messrs. Dallinger and 

 Drysdale of the existence of a contractile vacuole in this 

 monad, though they describe it in another. 



Their Heteromita, however, multiplied rapidly by fis- 

 sion. Sometimes a transverse constriction appeared ; the 

 hinder half developed a new cilium, and the hinder 

 cilium gradually split from its base to its free end, until 

 it was divided into two; a process which, considering 

 the fact that this fine filament cannot be much more 

 than 1 ^ of an inch in diameter, is wonderful enough. 

 The constriction of the body extended inwards until the 

 two portions were united by a narrow isthmus ; finally, 

 they separated and each swam away by itself, a com- 

 plete Heteromita, provided with its two cilia. Some- 

 times the constriction took a longitudinal direction, with 

 the same ultimate result. In each case the .process occu- 

 pied not more than six or seven minutes. At this rate, 

 a single Heteromita would give rise to a thousand like 



