Minute Anatomy of the Infusoria. 477 



middle size upwards, they may either become Acineta y or imme- 

 diately produce monad-like embryos. 



We may thus ideally arrange the different stages of develop- 

 ment through which the Vorticella pass : the largest end their 

 lives by becoming encysted ; the whole of the contents of their 

 bodies then passes into embryos, to which the dividing germ- 

 nucleus first gives origin. 



The embryos become fixed, develope from their posterior ex- 

 tremity a stalk, which is at first not contractile, and gradually 

 change their monad-like body into that of a common Vorticella. 



As soon as this has taken place, their very much smaller size 

 only distinguishes them from the perfect Vorticella. Even in 

 this imperfect condition they frequently multiply by continual 

 division and in a subordinate degree by external gemmation. 

 [This power of multiplication in the imperfect state, however, is 

 one of the most certain criteria that we have to do with an alter- 

 nation of generations.] By degrees the generations become larger 

 and larger ; certain individuals become encysted and pass into 

 the Acineta-form ; under this form they lead an independent 

 existence, but apply all their assimilated food to the re-formation 

 of the germ-nucleus, which they send forth from time to time in 

 the form of a common Vorticella. Finally, the last generation 

 become encysted, not to re-awake to an independent existence, 

 but to break up into a swarm of embryos. 



At last then, after many aberrations from the right track, 

 without however having previously observed wrongly, I had 

 found the complete history of development of the Vorticella, and 

 thereby set aside the observations of Pineau*, which, indeed, 

 bore the stamp of improbability about them. It becomes hereby 

 quite possible to explain the sudden appearance of Vorticella in 

 infusions, without assuming a generatio aquivoca, which has 

 been so boldly advocated lately by Dr. Grosf and Dr. Reisek %. 



For it is only necessary that a single Yorticella-cyst should 

 get into water previously free from Vorticella, in order to people 

 it in a very short time with troops of them. There can be no 

 difficulty in understanding how such a cyst may be imported, if 

 one considers that owing to their minute size, they are as readily 

 carried about by the air, as the spores of plants. A brisk wind, 

 ruffling the surface of some stagnant water, will as easily carry 

 away any Vorticella there may be on its surface, as particles of 



* Annates des Sciences Naturelles, 1 E 



t Siebold und Kolliker's Zeitschrift, BcL iii. p. 69. Dr. Gros did me the 

 favour to call on me at Berlin, but did not succeed in showing any thing 

 under the microscope by which his views might be justified. 



X Entwickelun^sgeschiehte des Thieres und der Pflanzen durch Urzeu- 

 gung. Berichte der Kaiserl. Aknd. der Wissenschai'ten ru Wien, 1851. 



