M.J. D'Udekem on the Metamorphoses of the Vorticclle. 7 



viduals, it is oftcu reniform, sometimes oval ; the surface of the 

 cyst becomes in some cases rough with little points. 



The cause of the encystment escapes us, like the causes of 

 almost all physiological phenomena. For its explanation we 

 may have recourse to two hypotheses : — either the animal is in- 

 duced to become encysted by the influence of an internal cause — 

 the phenomenon of encystment being then physiological, nor- 

 mal, and occurring necessarily; or the animal becomes encysted 

 from the influence of external agents — the phenomenon then 

 being abnormal and depending on chance. Of these two hypo- 

 theses the second appears the more probable ; indeed I have 

 always remarked that most of the Vorticellina become encysted 

 when the liquid containing them evaporates through exposure 

 to the air; moreover, we find a greater number of cysts in winter 

 than in any other season. I conclude from this that drought 

 and cold are two causes of encystment ; that these two causes 

 are probably not the only ones, but that there doubtless exist 

 others which it is more difficult to appreciate. 



The encystment of the Vorticellina appears to have a double 

 purpose : first, to withdraw these very delicate animals from the 

 destructive action of drought and cold; in the next place, to 

 allow them to undergo certain metamorphoses protected from 

 all external influences. 



2. Transformation of the Vorticellian, in the interior of the cyst, 

 into an Infusorium ciliated over the whole surface, and pre- 

 senting no orifice in its integuments (Opalina or Bursaria). 



The observations of M. Stein, M. Lachmann, and myself 

 agree in showing that the majority of the Vorticellina may re- 

 main in the interior of the cysts without undergoing metamor- 

 phosis ; they are then under the influence of a state analogous 

 to the hybernal sleep of the higher animals. When circum- 

 stances are favourable, that is to say, when humidity and a cer- 

 tain degree of heat are restored, they burst their envelope and 

 resume their former life. 



In M. Stein's work, he treats of several metamorphoses under- 

 gone by the Vorticellina in the interior of the cysts ; not having 

 observed these, I shall not speak of them, and shall occupy my- 

 self solely with that transformation which I have indicated at the 

 head of this section. 



The metamorphosis of a Vorticellian, in the interior of a cyst, 

 into an Infusorium ciliated over the whole surface, was for the 

 first time described by me in my memoir on the development of 

 Epistylis plicatilis. The following is the account I gave of the 

 phenomenon. 



The Epistylis plicatilis, in the interior of the cyst, becomes 



