1900-1.] Dr J. Y. Simpson on Binary Fission of Ciliata. 415 
presented no abnormality.* On the 17th there were four 
Paramecia on the slide, but one of them had developed a cleft 
"tail. The cleft was in a plane perpendicular to the dorso-ventral 
axis. From the first it extended to a depth of about 25 [x and did 
not grow deeper. Otherwise the creature appeared to be perfectly 
normal : the two contractile vacuoles functioned and the internal 
circulation swept round, clear of the divided tail. On the 18th 
it remained as it was. On the 19th it had divided, and the 
anterior half, though distinctly undersized and resembling rather 
the species aurelia in configuration, was yet normal in every other 
vespect and continued afterwards to divide by itself. On the 20th 
the original form again divided, but not on the 21st. However, on 
the 22nd it resumed operations, and while the anterior one still 
retained its aurelian characters, I noticed that the internal circula- 
tion of the posterior half no longer swept clear of the tail, but had 
partially entered into one (the dorsal) lobe, which now contained 
excretory granules and one or two small food vacuoles. At the 
same time this lobe had slightly increased in size, while the other 
had correspondingly decreased. On the 23rd it divided again, but 
not on the 24th. The anterior parts still retained the same 
characteristics as formerly, and gave rise themselves to otherwise 
normal Paramecia. In the posterior cleft-tail Paramecium the 
dorsal lobe continued to grow, while the other was more and 
more absorbed. The mouth also was driven unusually far back. 
On the 25th and 26th it again divided, but not on the 27th. The 
ventral lobe had now been completely absorbed, while the other 
had increased in size till it now measured some 60 /x ; there was, 
however, no proportionate increase in breadth, and there seemed to 
be a tendency for it to get blocked. At any rate the number of 
excretory granules increased, and the circulation slowed down. 
On the 28th and 29th it again divided, but the dorsal tail seemed 
thoroughly congested, and by the 30th it was dead. Hone of the 
daughters reproduced the peculiarity in themselves or in their 
descendants. 
The other case is still more peculiar. About the same time 
as the preceding exconjugates were isolated— one of several 
* No temperatures are given, as in this case they probably had no influence 
on the sequel. 
