MONADS 759 



mediately enters upon and passes through the same process of fission, 

 which from first to last is completed in from four to seven minutes ; 

 and being repeated at intervals of a few minutes, this mode of multi- 

 plication produces a rapid increase in the number of the monads. 



Such fission does not, however, continue indefinitely, for after 

 a successive series of fissions, followed in one of the divided bodies 

 for eight or nine hours, certain individuals do not again enter upon 

 the process of fission, but undergo a peculiar change, which shows 

 itself first in the absorption of the two lateral flagella and the great 

 development of the nucleus, and Afterwards in the formation of a 

 transverse granular band across the middle of the body (fig. 11, E). 

 One of these altered forms, swimming into a group in the ' springing ' 

 state, within a few seconds firmly attaches itself to one of them, which 

 at once unanchors itself, and the two swim freely and vigorously about, 

 shown in fig. 12, generally for from thirty-five to forty-five minutes. 

 Gradually, .however, a ' fusion ' of the two bodies and of their re- 

 spective nuclei takes place, the two trailing flagella of the ' springing ' 

 form being drawn in (fig. 13, F) ; arid in a short time longer the two 

 anterior flagella also disappear, and all trace of the separate bodies is 

 lost, the nuclei vanish, and the resultant is an irregular amoeboid mass 

 (fig. 14), which gradually acquires the smooth, distended, arid ' still ' 

 condition represented in fig. 14, a. This is a cyst filled with repro- 

 ductive particles of such extraordinary minuteness that, when 

 emitted from the ends of the cyst (fig. 15, a) after the lapse of four 

 or five hours, they can only be distinguished under an amplification 

 of 5,000 diameters, with perfect central illumination, i.e. the full 

 cone of a large-angled condenser. Yet these particles, when con- 

 tinuously watched, are soon observed to enlarge arid to undergo 

 elongation (figs. 16. 17. 18, 19, 20), and within two hours after their 

 emission from the sac the anterior flagellum, and afterwards the two 

 lateral flagella (fig. 19), can be distinguished. Slight movements then 

 commence, the neck-like protrusion shows itself, and in about 

 half an hour more the regular swimming action begins. About 

 four hours after 1 the escape of its germ from the sac, the monad 

 acquires its characteristic form (fig. 21), though still only one-half the 

 length of its parent : but this it attains 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 transformation 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 process ; 

 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 Ufa-cycle of this monad 

 lias 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 inonadine forms which they have studied. 



The bi-jlckgettcde or* ' acorn ' monad of the same observers (identi- 

 fied by Kent with the Polytoma uvella of Ehrenberg) presents some 

 remarkable peculiarities in its mode of reproduction. Its binary 

 fission extends only to the protoplasmic substance of its body, leaving 1 



