PEOPAGATION BY ENCYSTMENT. 47 



the parenchyma by which it is surrounded. The nucleus seems to 

 play an essential part in the fissuration ; for every time the animalcule 

 divides either longitudinally or transversely, this organ, which is usually 

 situated in the middle, divides also, so that each of the two new in- 

 dividuals has a nucleus. This nucleus, which is of a fine -granular 

 aspect and dense structure, retains its form when the animalcule is 

 pressed between two plates of glass and the other parts are spread out 

 in various ways. By direct light its colour appears pale yellow. 



(101.) A remarkable mode of reproduction, by encystment, observed 

 first by Stein in different species of Vorticella, appears to exist com- 

 monly among all the animalcules of this class*. The individual about to 



Fig. 21. 



Vorticella microstoma (Ehr.), showing different stages of the process of flssiparous reproduction. 

 The basis to which the group is attached consists of a finely granular mucous mass. 



become encysted contracts slightly, and closes its peristome ; and around 

 it there then appears a cloud-like sacculus formed by a viscid liquid, 

 which is probably the result of a cutaneous secretion. In this liquid 

 are developed granules, which, augmenting more and more in number, 

 and adhering together, finally form a membrane, which becomes hard 

 and resisting, although soft and flexible when first produced. This 

 encystment appears to have a double purpose : first, to withdraw these 

 very delicate animals from the destructive effects of drought and cold ; 

 and secondly, to allow them to undergo certain metamorphoses pro- 

 tected from all external influences. 



* M. J. d'Udekem, On the Metamorphoses of the Vorticellse," Ann. Nat. Hist. 

 for July 1859. 



