402 Proceedings of Royal Society of Edinburgh. [sess. 
creases only as the square, relief is obtained by the process. But 
it has been shown more than once — indeed is matter of common 
observation — that binary fission takes place at all stages in the 
development of certain Infusoria, and is not merely postponed till 
they reach a definite size. That is to say, binary fission is not 
necessarily connected with growth beyond the specific mass of the 
species. For example, in encysted forms, there is no possibility of 
growth previous to division, and in other cases, as the result of 
continuous division, there may be an actual decrease in size. To 
put the matter briefly, actual increase in size is neither a constant 
precursor or result of binary fission per se in the case of the Infusoria. 
All that can be said is that while in certain cases, e.g., Stylonichia, 
Euplotes, a distinct lengthening is noticeable at the commencement 
of the process, in others, e.g., Stentor , Spirostomum , no such phe- 
nomenon is observable. 
Another question has interest in this connection — where are the 
first signs of the process noticeable, in the nucleus or in the 
cytoplasm ? In view of the fact that both answers have been given 
by first-class workers, Butschli contents himself with stating for 
the majority that there are undoubtedly many instances where 
there are hints of new formations in the plasma, e.g., the “anlage” 
of the new ciliary apparatus, mouth, or contractile vacuoles, 
before any change in either macronucleus or micronucleus is 
observable. 
Simple binary fission, apart from nuclear considerations, is not 
a very complicated process. As already stated, the plane of 
division lies more or less at right angles to the long axis of the 
body, and usually approximately near the middle. It is necessary 
to make the qualification “more or less,” because in the case of 
Spirostomum teres , at any rate, the plane of division is somewhat 
oblique, as Stein observed so long ago as 1867 ( Der Orgctnismus 
der Infusionstliiere, Bd. II.). In the more highly organised 
Ciliata the special organs have to be duplicated, and this is 
achieved either by division of the already existing organ or by fresh 
formation in one of the offspring. The former method is compara- 
tively rare, and only occurs where the organ or system in question, 
as, e.g., the canals connected with the contractile vacuoles, runs 
practically the entire length of the creature. When the twin 
