GROWTH 519 



for other than intercomparison, the centrifugal force used should be 

 stated. The nomogram of Shapiro (1935a) simplifies this computation. 

 If the distribution of animals of different sizes changes, e.g., just after 

 a large proportion of them have divided or during endomictic reorgani- 

 zation, the total volume may not indicate the number present. Size 

 changes of yeast cells and failure to obtain constant packing have been 

 reported by Richards (1934). Shapiro (1935b) has discussed the valid- 

 ity of the centrifuge method with respect to marine ova. Simultaneous 

 counts and volume determinations of Colpidium campylum have been 

 made by Bond (1933). 



The population density of pigmented forms may be estimated from 

 the optical density of the suspension, by means of a nephelometer 

 (Richards and Jahn, 1933). A beam of light is passed through the 

 suspension and the amount of light absorbed by the organisms is meas- 

 ured by a photoelectric cell and a microammeter. When / is the micro- 

 ammeter reading with a given tube and medium and U is the reading 

 of the suspension at time /, then the optical density, D = log U — log I. 

 In this way the small variations in transmission of different test tubes 

 may be canceled out, and it is not necessary to open the tube, a factor 

 which may be important if the organisms are reared in a bacteria-free cul- 

 ture. 



The optical density depends on the number of organisms present, the 

 distribution of organisms of various sizes, and their metabolic condition. 

 It is sometimes difficult to relate measurements with this criterion to 

 the number of organisms present, because changes in internal cell struc- 

 ture (e.g., storage products) may alter their transparency. With proper 

 care and control, the nephelometer may give a useful measure of the 

 amount of protoplasm present in the population. For technical informa- 

 tion the following may be consulted: Kober and Graves (1915), Mestre 

 (1935), Russell (1937), and Muller (1939). Difficulties in the use 

 of the method have been summarized by Loofbourow and Dyer (1938) 

 and by Stier, Newton, and Sprince (1939). Miss Wright (1937) has 

 measured the turbidity of bacterial suspensions by passing the light 

 beam through the suspension at right angles to the axis of the photo- 

 electric cell. 



The dry weight of Protozoa may be obtained by filtering them from 

 the culture medium with filter paper of fine porosity, a sintered glass 



