458 



infusoria: — binary subdivision. 



This seems to be effected in the same general mode as the sub- 

 division of Protophyta ; and has been observed in many instances 

 to commence in the 'nucleus,' which may usually be distinguished 

 in the bodies of the Infusoria. The division takes-place in some 

 species longitudinally, that is, in the direction of the greatest 

 length of the body (Fig. 239, d, e, f), in other species, trans- 

 versely (Fig. 242, a, d), whilst in some, as in Chilodon cucullulus 



Fig. 240. 



A B C D F, F 



Fissiparous multiplication of Chilodon cucullulus: — A, b, c, suc- 

 cessive stages of longitudinal fission (?) ; d, e, f, successive stages 

 of transverse fission. 



(Fig. 240), it has been supposed to occur in either direction indif- 

 ferently ; but it seems most probable from recent discoveries that 

 what has been here supposed to be longitudinal fission (a, b, c) is 

 really an act of Conjugation (§ 347), and that the real fission is 

 transverse only (d, e, f). This operation is performed with such 

 rapidity, under favourable circumstances, that, according to the 

 calculation of Prof. Ehrenberg, no fewer than 268 millions might 

 be produced in a month by the repeated subdivisions of a single 

 Paramecium. When this fission occurs in Vorticella (Fig. 239), 

 one of the divisions is usually smaller than the other, sometimes 

 so much so as to look like a bud ; and this generally detaches itself 

 when mature from the main body, and swims freely about until it 

 developes a new footstalk for itself. But sometimes the two parts 

 are equal in size, and the fission extends down the stalk, which 

 thus becomes double for a greater or less part of its length ; and 

 thus a whole bunch of Vorticellae may spring (by a repetition of 

 the same process) from one base. In some members of the same 

 family, indeed, an arborescent structure is produced, just as in 

 certain Diatoms (Fig. 139), by the like processes of division and 

 gemmation. 



344. Many Infusoria at certain times undergo an Encysting 

 'process, resembling the passage of Protophytes into the 'still' 

 condition (§ 190), and apparently serving, like it, as a provision for 

 their preservation under circumstances which do not permit the 



