J. Y. Simpson 
401 
The species that I have specially examined in this connection are Paramoecium 
caudatum and Stijlonichia pustulata, but the data submitted in this paper deal 
only with Paramoecium. The points to which attention was directed were : — 
(a) the total length, 
(6) the extreme breadth, 
, . , • 1 breadth 
(c) the index r- . 
length 
In (a) differences in daughter forms were found ranging from 0 to 25yu., and 
in (6) from 0 to 20/i. A difference of from 1 to 20/a was also found in the 
distance between the two contractile vacuoles. 
It will be noticed that greater length in one member of a divided pair was 
not necessarily associated with less breadth. 
The following Table gives the lengths and maximum breadths in each of 100 
pairs of ParamoBcium caudatum, expressed in yu., the unit of measurement being 
5ll. It also gives the value of the index . 
° breadth 
The differences in general outline — e.g. one of the daughters was sometimes 
found to have a distinctive bend or curve in its contour — were best brought 
out by means of micro-photography. As micro-photography has not as yet been 
generally applied to living Ciliata, I give a few notes on the method employed. 
Ordinary Ilford chromatic plates were used, as also a Leitz 3 lens, which with 
the associated extension gave a magnification of about 80. The pictures were 
all taken instantaneously by incandescent light. The proportion of failures was 
very high, about 70 per cent. The chief difficulty was in obtaining a cell small 
enough to be wholly included within the magnification of the lens. Ultimately 
a block of soft paraffin was employed in which a hole was pierced with a fine 
needle. It was then sectioned with the microtome, and in this way by regulating 
the thinness of the section, a cell was obtained with the minimum of water in 
which the infusorian could live and yet be in focus all the time. A cover-glass 
was then superimposed, and as it was held in position by a generous application 
of vaseline round the edge, I was able to take the photographs in a horizontal 
position. Some of the photographs vvei'e sadly lacking in definition, but it is a 
matter of extreme dilSculty to calculate and adapt that amount of water in the 
cell which is sufficient for the free movement and life of the protozoon and yet 
is not too great to allow it to get out of focus during the exposure. 
An interesting question is raised as to the exact stage at which these 
measurements should be taken so as to represent this variation in its truest 
and greatest degree. It is very natural to suppose that for some little time 
after division the two daughter Paramoscia will be imperfectly formed, e.g. across 
the line of fission : we may imagine that the posterior end of the anterior 
daughter and the anterior end of the posterior one are not fully developed, as 
Biometrika i 43 
