1900 - 1 .] Dr J. Y. Simpson on Binary Fission of Ciliata. 407 
being fusiform, obtuse in front and thinner behind. This species 
is also credited with one micronucleus, and the zygote nucleus was 
said to give rise to eight corpuscles. The other species had a 
broader body, was almost oval, and obtuse at both extremities. 
It further possessed two micronuclei, and the zygote nucleus gave 
rise to only four corpuscles. Since this account no special notice 
seems to have been taken of the two species, except to cast doubt 
upon their existence as two distinct species. Thus, in the Zoologie 
Descriptive , Fabre-Domergue states that neither he nor Balbiani 
have ever come across this Paramecium with the double micro- 
nucleus, and he makes the remark in such a way as to suggest 
that Maupas was drawing on his imagination in his description of 
it. Accordingly we find bat one species — caudatum , with the 
single micronucleus — recognised generally in the text-books and 
other literature. There is no doubt, however, that these two 
distinct species do exist. Figs. 2 and 3 are photographs of the 
two species which give a very good idea of their relative sizes. 
Measurement of certain stained specimens which bring out the 
nuclear characteristics give P. aurelia a length of 80 y and a 
breadth of 40 y, while on the same scale P. caudatum has a 
length of 130 y and a breadth of 50 y. These figures, though 
hardly exact for the living form, bring out the peculiar feature 
of aurelia as compared with caudatum , viz., the high propor- 
tion that its breadth bears to its length. The magnification 
of the photographs is about 80. I may also mention here that 
I made frequent endeavours, through isolation of pairs, to get the 
two species to conjugate. The disproportion in size offered no 
a priori objection, as one often sees equal disproportion in the 
case of conjugating Stylonichia ; and even in the case of P. 
caudatum the inequality is often marked. The representatives of 
P. caudatum were selected from a culture in which an epidemic of 
conjugation had set in ; while the aurelia were taken from another 
culture which was far advanced in the number of its divisions. I 
never had the chance of contemporaneous epidemics amongst the two 
species, and accordingly always selected P. caudatum as the form 
that one certainly knew was ready for conjugation, inasmuch as it is 
the larger and probably more forceful species. Out of twenty-one 
attempts I had but two partial successes. Conjugation took place 
VOL. XXIII. 2 D 
