526 GROWTH 



During the first hour, Paramecia are Httle affected by chemicals, and 

 food plays no role (Estabrook, 1910). Increase in length is predomi- 

 nant at first, and then the animals fill out in breadth. Negative correla- 

 tions of length and breadth were found by Mizuno at one, four, and 

 six hours after fission. Erdmann (1920) found changes in the sizes of 

 Paramecia which were related to the endomictic rhythm. 



Individual growth has been measured in relatively few species, mostly 

 Infusioria. Data on other forms would be useful for comparison. Growth 

 studies should be made successively on the same living animals, to avoid 

 the difficulties of averaging and of using information from different 

 individuals. A promising method for obtaining the measurements would 

 be to take pictures of the animal in isolation culture, at frequent and 

 regular intervals, through the microscope, with a motion-picture camera. 

 The images could be measured from the film, which would provide a 

 permanent record for the analysis of the growth. 



The growth of individual Protozoa is quite similar to that observed 

 with other plants and animals. If we disregard the first decrease in 

 breadth, the growth curves are sigmoid. Sufficient information is not 

 available to indicate to what extent there is a general change of form 

 during growth. Many of these studies were not made with a constant 

 culture fluid. In order to compare the growth patterns of different species, 

 the culture conditions should be known to be optimal and reproducible. 

 Such studies would establish standards for the further study of environ- 

 ment on the individual growth pattern. The application of geometrical 

 and metabolic considerations, similar to those used by Schmalhausen 

 and Syngajewskaja (1925) with bacteria, will contribute to the theory 

 of growth. 



The Growth of Colonial Protozoa 



The colonial Protozoa are believed by Faure-Fremiet (1930) to 

 constitute an intermediary step between the unicellular and the multi- 

 cellular organisms. The colony grows by regular division from a free- 

 swimming cell, until a size characteristic for the species is reached. When 

 the environment is unfavorable, the growth is restricted. In favorable 

 cultures two types of growth are found. In certain species, e.g., Epistylis 

 arenkolae and E. Perrieri, the first divisions are dichotomous and equal, 

 and the mass growth of the colony follows in geometrical progression. 

 Later, the sister cells divide unequally, and the growth becomes arith- 



