280 THE BEGINNINGS OF LIFE. 



The pellicle that forms at first is, however, not 

 always persistent : after twenty-four or thirty-six hours 

 it may sink to the bottom, whilst another gradually 

 takes its place which may prove more durable. It is 

 not very plain why some pellicles break up and sink 

 in this way, but it would seem very probable that such 

 an occurrence may be associated with an imperfect 

 secretion or formation of that transparent jelly which, 

 in ordinary cases, so much helps to give it coherency 

 and strength, and whose presence is probably as ne- 

 cessary in order that subsequent evolutional changes 

 may ensue. In some infusions or fermentable solutions, 

 however, no distinct pellicle is ever formed. Flocculi 

 may appear in the clouded liquid, which, after a time, 

 sink to the bottom of the vessel ; or, without the 

 formation of flocculi, a deposit gradually accumulates, 

 whilst the previously clouded supernatant liquid be- 

 comes more or less clear. 



Occasionally it happens that the substance of a 

 pellicle may be almost wholly composed of minute 

 Torula cells Bacteria being well-nigh absent. I once 

 saw a very remarkable instance of this in an infusion 

 of turnip. In certain of the cases, also, in which no 

 distinct pellicle forms, the fine sediments or flocculi 

 which gradually collect at the bottom of the vessel 

 more especially when the infusion has an acid reaction 

 are found to consist either 1 wholly or largely of vegetating 

 Torula cells. 



1 In two or three cases I have failed, after a long search, to find a 

 single Bacterium amongst the myiiads of Torula cells. 



