404 Proceedings of Eoyal Society of Edinburgh. [sess. 
the old ones, which are generally lifeless. Their area of origin is 
very much less than that which they eventually occupy. The new 
anal cirri, in the case of the posterior daughter, are formed above 
the old. The area occupied by the latter is marked off by a sort 
of furrow, and undergoes degeneration although there is no actual 
separation from the body of the creature — rather absorption. 
In binary fission the nucleus is naturally the seat of the most 
complicated processes. Even when changes are first noticeable in 
the cytoplasm, it can be sufficiently well maintained on a priori 
grounds that there are previous changes in the nucleus, which, 
though invisible, are yet the inciting cause of those that are 
visible in the plasma. Macronucleus and micronucleus alike 
divide, the latter usually in advance of the former. It is not yet 
possible to state authoritatively whether, in the case of two micro- 
nuclei, the two halves of the same micronucleus go to one 
daughter, or whether it is a half of each of the micronuclei that go 
to form the daughter micronucleus in any one of the offspring. 
With regard to the duration of the process, no definite rule can 
be laid down. I have noted the following periods for the forms 
named when under observation : — 
Paramecium caudatum , 1^-2 hours. 
Stylonichia pustulat a, 1-2 hours. 
Lacrymaria olor , 1-1J hours. 
Spirostomum ambiguum , 1-2 hours. 
Spirostomum teres , 1-2 hours. : 
As regards the rate of fission, it may be noted in the first place 
that it is by no means constant throughout the Ciliata, but varies 
with the species. Each species has its own normal rate of division 
depending upon its specific qualities. The following list of rates 
of division is based upon Maupas. I have ventured to modify 
it slightly, in accordance with my own results. The range of 
temperature on which I base the modifications is 16°-22° C. 
Stylonichia pustulat a, every 12-16 hours. 
Euplotes patella, every 24 hours. 
Onychodromus grandis, every 12 hours. 
Oxytricha pellionella , every 8 hours. 
Spirostomum ambiguum , every second day. 
Spirostomum teres, every two or three days. 
