256 The Philippine Journal of Science ms 
I have used a procedure which is an adaptation of that used 
by Cohn for estimating the number of cells in the coenobia. 
With a Zeiss acromatic objective D and a net-ruled ocular-mi- 
crometer adjusted so that each small square measured 18 microns 
on a side, an area of the object was selected which occupied 25 
of the small squares, preferably 5 by 5. When shadows of 
daughter coenobia or other objects interfered with the use of a 
square area, groups of small squares in various arrangements 
were taken. When a good view of so large an area could not be 
obtained, a smaller area was used and the number counted mul- 
tiplied by the factor required to give the number of cells in 25 
small squares. Areas were taken as nearly central as possible, 
to keep down the error arising from projection of the spherical 
surface into the image plane. In cases where the intercellular 
distance decreased from anterior to posterior pole, either an 
equatorial, area was used, or, in cases of polar presentations, a 
count was made around each pole and the average of the two 
counts taken. The count represented, then, the average number 
of cells in an area 90 by 90 microns square, equal to 8,100 square 
microns. The counts were made by inspection when convenient, 
otherwise by making a camera drawing of the boundaries of the 
area and the outlines of the included cells. Cells on the boundary 
were included when their larger parts lay within the boundary. 
The diameter of the coenobium was measured with an eyepiece 
micrometer, or with a camera lucida measuring scale prepared 
by making a camera lucida drawing of a stage micrometer. The 
latter is usually the more convenient way. 
To use these data let d be the mean diameter of the coenobium 
in microns, a the area of the count in square microns, and n the 
number of cells counted. Then the formula for the number of 
cells, N, will be ; 
d 2 n 
N = . 
a 
TT 
Using the same area for all counts, or reducing all counts to 
correspond to that area, the formula put in the form : 
N = 
n 
a 
d\ 
TT 
may be reduced, for the area 8,100 square microns, to: 
