278 THE BEGINNINGS OF LIFE. 



surface of the solution, and, becoming motionless, 

 to form a very densely aggregated but pretty uniform 

 layer of a more or less granular appearance, consti- 

 tuting the so-called c proligerous pellicle.' But even 

 the simplest pellicle is not constituted solely by the 

 mere aggregation of these bodies. As pointed out by 

 Cohn 1 , the organic particles are surrounded by, and 

 imbedded in, a thin pellucid and almost invisible jelly- 

 like stratum, which is best revealed, in a microscopical 

 specimen, after the addition of a drop of a dilute 

 aqueous solution of iodine. The gelatinous matter 

 is not coloured by this reagent, and is thus rendered 

 apparent. 



The pellicle gradually continues to increase in thick- 

 ness, and owing to the additions being made from 

 below, its under surface frequently becomes very 

 irregular, from the presence of numerous bosselated 

 projections. As fast as the Bacteria and plastide- 

 particles accumulate, they appear to become surrounded 

 by the almost invisible gelatinous material above re- 

 ferred to. In this condition they are motionless, and 

 it has been assumed, without sufficient proof, by 

 Pouchet and most of the other heterogenists, that they 

 were also dead. Bacteria which are really dead, how- 

 ever, do not become enveloped in such a material. 

 The observations of Cohn, which I have frequently 

 confirmed, show that the Bacteria again begin to move as 

 soon as any of them may have been set free from the 



1 'Entwick. Geschichte der Mikros. Algen und Pilze, 1854.' 



