222 A Biometncal Study of Conjugation in Paramecium 
is a difficult character to measure, principally for the reason that the line of union 
between the two individuals is in many cases somewhat indistinct, and it thus 
becomes difficult to decide at just what point the body of one individual ends 
and that of the other begins. Furthermore, the close application of the oral 
surfaces of the bodies of the conjugants to each other in all but the earliest 
stages of the conjugation process changes somewhat the shape of a cross section 
of the body. This change is in the direction of a flattening of the oral (ventral) 
side of the body. Since for obvious reasons in the case of the conjugants the 
longest dorso-ventral diameter in the sagittal plane had to be taken as the breadth 
dimension, it is clear that any tendency towards flattening of the oral contour 
will appear directly in the measurements. The result of such a tendency will 
be to make the mean breadth lower, relatively to the length, in the conjugants 
as compared with the non-conjugants. As will be seen later, this is exactly what 
the measurements show. The conjugants are, in proportion to their length, 
narrower than the non-conjugants. For the reasons just stated, no special weight 
must in the opinion of the writer be laid on the breadth dimensions of the 
conjugants. The difficulties in the case were perfectly apparent while the 
measurements were being made, and the breadth dimensions for the conjugants 
were only included as a matter of routine, and because there was some curiosity 
to see how the constants would turn out. As a matter of fact the results are 
much better, i.e., more regular and consistent from series to series, than was 
expected after the appreciation of the difficulties in the case which one gets 
from seeing and measuring the actual specimens. It should be kept distinctly 
in mind, however, that much weight cannot be laid on the results from the 
breadth measurements of conjugants. 
In both the conjugant and non-conjugant individuals the point on the long 
axis of the body where the line of greatest breadth crossed it had, of necessity, 
to be estimated by the eye, but, owing to the shape of the body, this usually 
gives no difficulty in practice. In a normal Paramecium the outline of the body 
slopes off rather sharply from the region of the greatest breadth to the posterior 
end. The region of greatest breadth varies somewhat in relative position in 
different individuals and in different samples. It usually falls slightly behind the 
dimension is in consequence the same whatever side of the animal happens to be uppermost. This 
I take to be a fact known to everyone who has more than the most superficial objective acquaintance 
with Paramecium. There are verj' many ways in which it can be demonstrated. One very simple 
way is to bring a living Paramecium which is rotating on its long axis into such position that its 
maximum breadth dimension falls between two cross hairs of an ocular micrometer. Another way 
is to observe with a Braus-Drtiner microscope a living Paramecium standing on end, so that one gets a 
direct view of the posterior end. It will then be seen that the outline at the plane of greatest breadth 
is a circle. Finally if Mr Lister will look at an actual cross-section (1 to 5 microns thick) of the body of 
a Paramecium he will see that in the region of the greatest breadth the sections are circular in outline. 
With his notions of the limitations of technique Mr Lister may doubt the possibility of making these 
sections, but I have studied a long series of such sections of Paramecium. It seems a very trivial thing 
to say in a technical paper, but apparently it needs to be pointed out to Mr Lister at least that the 
asymmetry of Paramecium is the result of the asymmetrical course of the oral groove. Posterior 
to the oral groove the animal is entirely symmetrical. K. P. 
