Size as Structure and Function 3 



an examination of the data, heretofore scattered, that 

 have been obtained by measuring unicellular organ- 

 isms. While the data to be actually analysed are 

 limited to one class of organisms, it is fairly certain 

 that conclusions reached will be applicable to many or 

 all kinds of organisms or their constituent tissues and 

 cells. Moreover, some problems of size, such as the 

 fundamental one of the cessation of the increase in 

 size at maturity, can best be studied upon organisms 

 such as unicellular ones whose inhibitors of growth 

 are chiefly outside their bodies. 



The problem in protista. Among most species of 

 unicellular organisms the question of "how big" is 

 presented in an extraordinarily simple way. A typical 

 individual spends definite amounts of time and of activ- 

 ity in gaining to itself more body substance; it grows. 

 Then it divides into two individuals. How is the point 

 of time set at which the individual has gained sub- 

 stance enough to become two individuals ? Or, in other 

 species, an individual of adult size forms a protuber- 

 ance or bud as it accumulates living material in excess 

 of its own natural size, until the material in the bud is 

 equal in bulk to the parent, when separation into two 

 adults occurs. What keeps the size of the parent from 

 increasing within itself ; and why does separation occur 

 upon the attainment of the specific size? 



It must first be ascertained just how precisely the 

 adult size is controlled. An estimate of this precision 

 is to be obtained by measuring under the microscope 

 (with an ocular micrometer) a considerable number 

 of individuals. In a certain sample of trypanosomes, 

 the uniformity of body length shown by the frequency 

 curve of figure 1 was found. In this case the individual 

 trypanosomes were automatically selected as adults, 

 for Taliaferro ('23) observed that, as these trypano- 

 somes were cultured in the body blood of a rat, their 



