1900-1. ] Dr J. Y. Simpson on Binary Fission of Ciliata. 401 
Observations on Binary Fission in the Life-History of 
Ciliata. By Dr J. Y. Simpson. (With Two Plates.) 
(Read June 3, 1901.) 
The simplest and most common form of reproduction amongst 
the Ciliata is binary fission. In this ordinary, possibly vegetative, 
method of reproduction the plane of division is generally perpen- 
dicular to the long axis of the creature, To this generalisation the 
Yorticellidse form an apparent exception, but on the view that their 
evident long axis really corresponds to the dorso-ventral axis of 
other ciliates their case falls into line with that of the rest of the 
sub-class. Formerly many instances of fission in the direction of 
the long axis were described ; they may safely be considered to 
have been mere instances of conjugation. 
Binary fission most commonly takes place while the creature 
moves about; i.e., it is (in most cases at least) an activity 
temporarily added to all the other activities of ciliate existence. 
As such it may be considered to be the original method of repro- 
duction. Under other conditions fission may take place when the 
creature is at rest ; or, in other words, in certain cases binary fis- 
sion is not associated with free movement ; on the contrary, this 
stationary fission is usually associated with the formation of a cyst. 
Under these circumstances the operation may take place more than 
once in succession. Such stationary fission together with budding — 
which is simply a form of fission where the products are so unlike 
in size as to be distinguishable as parent and offspring — are best 
considered as modifications of ordinary binary fission. 
By ordinary binary fission, then, we understand the division of 
a ciliate during its active free existence into two daughters by a 
constriction more or less transverse to its long axis. It is con- 
fessedly difficult to arrive at a rationale of binary fission. One 
might suppose that it was associated with a certain limit of size, and 
that, as is more evident in the case of globular Rhizopoda, since the 
bulk increases as the cube of the diameter while the surface in- 
