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206 LEEUWENHOEK AND HIS “‘ LITTLE ANIMALS ”’ 
These observations caused me to view more nicely the 
lesser animalcules that were also swimming about in the 
water, and whereof the number was a good twenty times 
as many as that of the little animals aforesaid. Some of 
these animalcules were also coupled; and I saw that 
these likewise not only stayed long a-copulating, but also 
that one of the pair, whether it swam through the water 
or ran upon the glass, dragged the other one forward, or 
trailed it after itself. 
The foregoing passage is of great interest, since it is 
evidently a record of observations made upon the conjugation 
or fission of ciliates. In my opinion the evidence is in favour 
of the view that Leeuwenhoek, on this occasion, witnessed 
both these phenomena; though it is regrettable that he 
gave so brief an account of what he saw. I cannot help 
thinking that what he interpreted as “one animalcule 
dragging another one along’ was a conjugating pair; whereas 
‘one animalcule trailing another behind it”’ was really an 
organism dividing transversely into two. Later,’ as we shall 
soon see, he described the conjugation of ciliates in unmistak- 
able terms.” 
In a letter written some 3 years afterwards, Leeuwenhoek 
mentions the discovery of “animalcules”’ in a new situation. 
After describing his observations on the oyster, and its 
generation, he remarks * : 
I also paid attention to the ordinary water that is in 
the shells of oysters; and I discovered therein a great lot 
of little animalcules, which, in bigness and figure, were 
like the little animalcules that you generally find in 
canal-water, rainwater cisterns, and common ditch-water. 
These animalcules were, in my judgement, above five 
hundred times smaller than a young oyster. 
' Cf. also the earlier observations in Letter 33, p. 200 supra. 
> Letter 96, p. 213 infra. 
* Letter 92. 15 August 1695. To Frederik Adriaan, Baron van Rhede. 
Published in Brieven, Vijfde Vervolg, p.114: Opera Omnia, Vol. II, p. 511. 
No MS., and not in Phil. Trans. 
