490 Dr. A. Gruber on the 



when from any circumstances the pool, brook, &c. was nearly 

 dried up ; the regularity with which the divisions follow one 

 another in time is not thus explained, and tJiis can apparently 

 only he the expression of a constantly acting internal law. 



The absence or jpresence of nutritive material for the Stentors 

 was in all these experiments without influence upon the time of 

 the division. I had isolated animals in watch-glasses con- 

 taining nearly pure water, and in others in which the water 

 swarmed with Parama;cia (a chief food of the Stentors) and 

 other Infusoria ; but in both the multiplication went on in the 

 same manner, and indeed always so that the animals did not 

 grow between two divisions, and therefore lost in volume from 

 one division to the next. I have frequently made measure- 

 ments of the individuals under experiment before isolation, 

 measuring them while swimming about, when they present a 

 mean state of extension ; then the daughters, granddaughters, 

 &c. were also measured, and it was found that the volume 

 decreased to about one half, then to a quarter, and so forth. 

 I say about, because the animals produced by division ap- 

 peared a little larger than the corresponding fragments, which 

 may be due probably to inception of water. The Stentors 

 which I isolated were generally nearly of the same size, and 

 they divided only to the third generation, so that the last 

 generations in these experiments were always nearly of the 

 same dimensions. 



If I isolated smaller animals, they divided only to the 

 second generation, which also again showed the smallest 

 measurement. 



I believe there is no doubt that in these phenomena a nor- 

 mality is expressed, and that we have not to do with the 

 products of accident. Even in the aquaria in which the 

 colonies of Stentor live under natural conditions of existence 

 we often find the Infusoria of very small average size, and it 

 may very well be that these had just been subjected to a rapid 

 sequence of divisions. / believe that among the Infusoria we 

 may distinguish txcohinds of spontaneous division^ one of which 

 occurs when the individual by growth has attained a certain 

 size which cannot be exceeded ; this is the multipHcation which 

 has been characterized as the growth of the individual beyond 

 the prescribed measurement. A second mode of increase is by 

 divisions following upon one another rapidly and in definite 

 intervals of time without intervening growth^ and therefore 

 combined with continual decrease of the size of the body down 

 to a definitive smallest measurement. This latter mode of mul- 

 tiplication, of the existence of which I have already furnished 

 proof, will occur when the Infusoria are placed under un- 



