MICROSCOPIC FORMS OF ANIMAL LIFE. 29 



its characteristic form (fig. 20), though still only one-half the length of 

 its parent; but this it attains (passing through the stage shown in fig. 21) 

 in another hour, and the process of multiplication by fission, as already 

 described, commences very soon afterwards. There can be no reasonable 

 doubt that the * conjugation ' of two individuals, followed by the trans- 

 formation of their fused bodies into a sac filled with reproductive germs, 

 is to be regarded (as in protophytes) in the light of a true generative pro- 

 cess; and it is interesting to observe the indication of sexual distinction 

 here marked by the different states of the two conjugating individuals. 

 There is every reason to believe that the entire life-cycle of this Monad 

 has thus been elucidated; and it will now be sufficient to notice the 

 principal diversities observed by Messrs. Dallinger and Drysdale in the 

 life-cycles of the other Monadine forms which they have studied. 



419. Their simple uniflagellate Monad (Nonas Dallingeri, Kent), 

 having an ovate form with a long diameter never exceeding l-4000th of 

 an inch, and advancing slowly with a straight, uniform motion like that 

 of Monas termo, differs from the preceding in its mode of multiplication; 

 for this takes place, not by duplicative fission, but by the breaking-up of 

 the sarcodic substance (as in the production of f swarm spores' by Proto- 

 phytes) into from thirty to sixty segments, which, at first lying closely 

 packed together, make their escape as free-swimming Monads, each pro- 

 vided with its flagellum. Conjugation, in this type, occurs between the 

 ordinary forms and certain individuals distinguished between their some- 

 what larger size, and by the granular aspect of their sarcode towards the 

 flagellate end; and there is reason to think that the latter have never un- 

 dergone the segmentation by which the former have been multiplied. 

 The smaller are absorbed, as it were, into the larger; and the latter passes 

 after a time into the encysted state, corresponding in its subsequent his- 

 tory with the preceding type. The bi-flagellate or 'acorn' Monad of 

 the same observers (identified by Kent with the Polytoma uvella of Ehren- 

 berg) presents some remarkable peculiarities in its mode of reproduction. 

 Its binary fission extends only to the protoplasmic substance of its body, 

 leaving its envelope entire; and by a repetition of the process, as many 

 as 10 segments, each attaining the likeness of the parent, are seen thus 

 inclosed, their flagella protruding through the general investment. 

 This compound state being supposed by Ehrenberg to be the normal one, 

 he named it accordingly. But the parent-cyst soon bursts, and sets free 

 the contained ' macro-spores,' which swim about freely, and soon attain 

 the size of the parent. Again, the posterior part of the body of certain 

 individuals shows an accumulation of granular protoplasm, giving to that 

 region a roughened acorn-cup-like aspect; the bursting of the projection, 

 while the creature is actively swimming through the water, sets free a 

 multitude of shapeless granular fragments, within each of which a 

 minute bacterium-like corpuscle is developed; and this, on its release, 

 acquires in a few hours the size and form of the original monad. This 

 process seems analogous to the development of ' micro-spores ' among 

 Protophytes, by the direct breaking-up of the protoplasm. It is, like 

 the previous process, non-sexual or gonidial; the true generative process 

 consisting here, as in the preceding cases, in the ( conjugation ' of two in- 

 dividuals, with the usual results. 



420. A Cercomonas (C. typicus, Kent), characterized by the posses- 

 sion of a flagellum at each end, was found to multiply, during eight days 

 (and nights) of continuous observation, by transverse duplicative subdivi- 

 sion alone. But certain individuals then exhibited a remarkable change, 



