50 GENERAL HI3T0BV OF 



duction by gemmation ; but regards the vi\'iparity of Ilonas vivipara 

 as extremely doubtful, and tlio formation of ova in any Infusoria 

 and their disposition, as far from established. He would, moreover, 

 look upon the appearances observed and considered by Ehrenberg as 

 indicative of opposition, merely as instances of the phenomenon of 

 diffluence, or of the breaking up of the gelatinous substance {corcode) 

 of the animal into spherical atoms, from want of moisture. 



Siebold agrees with Dujardin, in thus limiting the process of repro- 

 duction to the two forms of gemmation and of spontaneous fission, 

 and from this hypothesis of the non-production of oA^a by Inftisoria, 

 argues the absence of true generative organs. The appearances 

 interpreted by Ehrenberg as evidences of the presence of ovaries, 

 testes, and vesiculoe seminales, afford, according to Siebold, no ground 

 for the assumption of such functions. 



The body, presumed by Ehrenberg to be the testis of Polygastrica, 

 is generally found occupying nearly the centre of the animalcule, 

 and is readily distinguished from the surrounding loose parenchj-ma, 

 by its more solid granular character, its clear outline, and generally 

 by its dull yellowish coloiir. 



Although the office of this body in secreting a spermatic fluid may 

 be justly called in question, direct observation being contrary to it, 

 yet this so-called testis, or, perhaps, more correcth^, this nucleus, 

 certainly plays a most important part in the well-observed mode of 

 propagation by spontiineous fission; for whenever fission, whether 

 longitudinal or transverse, is about to occur in an animalcule, the 

 first change observed is a progressive constriction of the nucleus, 

 succeeded by that of the body generally. This constriction goes on 

 till division is complete, each segment of the body being consequently 

 provided with a nucleus. The di-vision of the nucleus, as an essential 

 element iu the process of spontaneous fission, may be well observed 

 in the transverse division of Parameeium, Bursaria, or Chihdon. 



Sometimes, two, or even more nuclei, are met with in animalcules, 

 as in AmpTiUeptus anser and A. fasciola (figure 314), in StylonycMa 

 Mytilus, in Stentor coeruletfs, &c. 



The nucleus, from the circumstance of being the last part to suffer 

 death, remaining unchanged when the destruction of the surrounding 

 parenchyma is rapidly advanciiig, may be supposed to partake of a 

 higher dcgi'ce of vitality. 



