550 THE NATURAL HISTORY OF 



pear in a given infusion, at first exclusively, then princi- 

 pally; they gradually diminish, become more and more 

 rare, and finally disappear altogether. After some time 

 their number again increases, and reaches, as before, an 

 incredible amount ; and this proceeding may be repeated 

 several times. Thus a glass which at one time pre- 

 sented only still forms, contained some vs^eeks before 

 nothing but motile ones, and would again in a few weeks 

 contain nothing else. 



The same thing may be observed with respect to the 

 segmentation. If a number of motile cells are transferred 

 from a larger glass into a small capsule, it will be found, 

 after the lapse of a few hours, that most of them have 

 subsided to the bottom, and in the course of the day, 

 they will all be observed to be on the point of sub- 

 division. On the following morning the divisional gene- 

 ration will have become free ; and on the next, the bottom 

 of the vessel will be found covered with a new generation 

 of self-dividing cells, which again proceed to the forma- 

 tion of a new generation, and so on. This regularity, 

 however, is not always observed. 



The influence of every change in the external conditions 

 of life, upon the propagation, is highly remarkable. It 

 is only necessary to pour water, from a smaller into a 

 larger, shallower vessel, or one of a difierent kind, at 

 once to induce the commencement of segmentation in 

 numerous cells. The same thing occurs in other Algse ; 

 thus the Vcmcherioi almost always develope zoospores, 

 at whatever time of year they may be brought from their 

 natural habitat into a room. 



Light is conducive to the manifestation of vital action 

 in the motile zoospores, and they always seek after it, 

 collecting themselves at the surface of the water, and at 

 the edges of the vessel. 



But, in the propagative act, on the contrary, and 

 when they are about to pass into the still condition, the 

 motile Protococcus-cells seem to shun the light ; at all 

 events they then seek the bottom of the vessel, or that 



