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Recent Progress in our Knowledge of 
the nucleus of the Infusoria performs the function of a reproductive 
organ, though the opinions entertained as to the mode in which it 
thus acts are extremely divergent. 
It is now admitted that in the reproduction of unicellular 
organisms both in the animal and vegetable kingdom, the nucleus 
takes an important part, and by its division as a primary act ushers 
in the division of the rest of the protoplasm. Even in the cells 
which form constituents of tissues, the part played by the nucleus 
is altogether similar, its division always preceding the division of 
the cell itself. 
In quite a similar way does the nucleus behave in the ciliate 
Infusoria. The non-sexual reproduction of the Infusoria by division 
is perhaps universal. In such cases the division always begins by 
the spontaneous halving of the nucleus, and this is followed by a 
similar division of the surrounding protoplasm, exactly as in the 
ordinary simple cell. 
Another phenomenon in which the nucleus plays an important 
part is named by Haeckel “ spore formation.” Under this designa- 
tion he comprehends all those cases in which — the idea of a previous 
fecundation being rejected— the nucleus breaks into numerous 
pieces, and each of these, apparently by becoming encysted in a 
portion of the protoplasm of the mother body, shapes itself into an 
independent cell — a so-called germ-globule ( Keimkugel ). Now this 
is a true spore — just as much so as the spores which arise quite in 
the same way in unicellular plants. The whole process is to be 
regarded as a case of the so-called endogenous multiplication of cells. 
Most authors, however, take a different view of the nucleus. 
Following Balbiani, they regard it as an ovary ; and to the frag- 
ments into which it breaks up they assign the significance of eggs ; 
while the so-called nucleolus, which lies outside the nucleus, is, as 
we have seen, believed to be a testis in which spermatozoa are 
developed for the fecundation of the eggs. 
We must bear in mind, however, that this “nucleolus” has 
been hitherto found in but a disproportionately small number of 
species, while the spermatozoal nature of the apparent filaments 
which have been noticed in it has by no means been proved ; and 
we have already seen that some observed facts, such as those 
adduced by Butschli, are opposed to the view which would assign to 
them the nature of true spermatozoa. 
As Haeckel remarks, however, even though the so-called 
nucleolus be really a testis fecundating the eggs or fragments 
derived from the breaking up of the nucleus, this would afford no 
valid argument against the unicellularity of the Infusoria, for pre- 
cisely the same sexual differentiation and reproduction are found in 
unicellular plants. 
It may now, then, be regarded as proved that the process by 
