346 GENEEAL HISTORY OF THE INFrSOEIA. 



some would call it, fissation, and by gemmatioii or budding, which, from not 

 being true generative acts, have been called ' vegetative ' modes of propagation 

 or multiplication. Kecently, however, the Ciliata have had attributed to them 

 true generative processes, resulting in the development of embryos either with 

 or without intercurrent metamorphoses. 



The simpler processes of fission and gemmation are, in Stein's opinion, modes 

 of propagation pecuhar to immatiu-e beings, and are replaced in mature 

 animalcules by the agency of germs or embiyos. 



Fissiojsr. — This duplicative subdivision may be longitudinal, transverse, or 

 obhque ; and whilst some species divide in only one dii'ection, others are 

 capable of so doing in two, for instance, in the longitudinal and transverse, 

 but not simidtaneously. Among the VortkelUna longitudinal fission alone 

 occurs ; Paramecium (XXIX. 27), Chilodon, and others divide both longi- 

 tudinally and transversely; Lacjenophrys obliquely only. Fission has not 

 been mtnessed in SjiirocJiona nor in Trichodina, nor in Coljjoda when in a 

 free state and not encysted. 



Ehrenberg came to the conclusion that multiplication by spontaneous divi- 

 sion is the character which separates animals from j)lants. It is true (he 

 argued) that gemmation in plants, especially in veiy simple cells, is at times 

 very similar to the division in animals ; but this relates to the form, not the 

 formation. A vegetable cell, apparently capable of self-division, produces 

 one, or contemporaneously many exterior buds (gemma;), vrithout any change 

 in its interior. An animal which is capable of di\ision, first doubles the inner 

 organs, and subsequently decreases exteriorly in size. Self-division proceeds 

 from the interior towards the exterior, from the centre to the periphery' ; 

 gemmation, which also occurs in animals, proceeds from the exterior towards 

 the interior, and forms first a wart, which then gradually becomes organized. 



This supposed distinction between fission in vegetable cells and that in 

 simple animals like Infusoria is set aside by modern researches, which show 

 that, when a plant- cell is about to divide, the mucilaginous layer of the wall 

 (L e. the primordial utricle) manifests a constriction, which presently involves 

 the waU itself, and, gradually deepening, at length cuts the ceU into two. The 

 observations on this subject in the chapters on Desmidie^ and Diatome^ will 

 more completely elucidate it. 



Considered with respect to the condition of the animalcule, fission occurs 

 in the active and unchanged state, as in Paramecium ; or In a contracted state, 

 as in Vorticellina ; or only when encysted, as in the case of Colpoda. Hence 

 it follows, that it presents several slight modifications in its course. One 

 general fact is, that whilst fission proceeds, the rotation of the contents of the 

 animalcule is at a stand- still. In its simplest variety, the dividing being first 

 presents a constriction at each pole or side of the body, which gradually ex- 

 tends until it completely cuts it into two equal or unequal parts. Simulta- 

 neously Avith the fh'st indication of an act of fission, and in some cases before 

 a sign of it is to be detected in the peripheiy of the animal, it has been 

 generally taught that the nucleus, after elongating and usually disposing 

 itself across the direction of the line of scission, takes the initiative in the 

 act, by commencing a fission of its own substance (XXIX. 27), which sub- 

 sequently proceeds step by step with that of the entire body, until complete. 

 This statement is, according to Lachmann {A. N. H. 1857, xix. p. 230), a 

 mistake when made respecting the Protozoa generally ; for in some cases the 

 division of the nucleus is consecutive to that of the body, and " in others, 

 again, the actual fissation of the nucleus does not lead to that of the body, 

 but embryos are developed in it ; " on the other hand, ^' fissation is generally 

 commenced rather bv a new formation of contractile vesicles." 



