256 The Philippine Journal of Science 1918 



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 ; 



<P n 



N = — - . 



Using the same area for all counts, or reducing all counts to 

 correspond to that area, the formula put in the form : 



may be reduced, for the area 8,100 square microns, to: 



AT _ rf2_ 



2578 



