448 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



therefore, to outline the main features of the growth oi Paramecium, 

 and to construct curves which shall give an idea of the processes 

 involved. In spite of an incredible amount of work devoted to col- 

 lecting the data, certain of the less important features of the growth 

 curves must remain obscure, but the main facts are clear. 



The main outlines of the changes due to growth are as follows : 

 From the time the constriction appears in the mother until a few 

 minutes after separation takes place, the length increases rapidly, 

 while the breadth decreases a little. A few minutes after separation 

 the processes become less rapid. The breadth soon reaches its mini- 

 mum, then begins to increase like the length, though more slowly. 

 Growth in length continues for at least eighteen hours ; the time 

 undoubtedly varies with the conditions. The breadth continues to 

 increase for some time, but it undergoes marked fluctuations, due to 

 environmental conditions. In lot 10 (Table X.) it decreased between 

 the ages of 12 and 18 hours ; this is probably an environmental effect, 

 not one due to the normal growth processes. 



As the time for fission approaches the animals are considerably 

 more than twice as long as the original halves from which they devel- 

 oped. Now as fission comes on they shorten and thicken, all tending 

 to approach a uniform length and thickness. There is thus much 

 less variation in the dimensions at the beginning of fission than in 

 specimens taken at random. Now the constriction appears and the 

 animal begins to narrow and extend in the way already described, 

 finally separating into two parts. 



If from our data we construct curves showing these changes, we 

 get such results as are shown in Diagram 5. 



Method of Constructing the Curves. — The horizontal scale repre- 

 sents the time in hours, while the vertical scale represents the meas- 

 urements of the animals in microns. The upper curve shows the 

 length, the lower one breadth, as measured from the base line. 

 Fission is assumed to take place once in twenty- four hours, which is 

 an approximation to a rate commonly occurring. The time between 

 the appearance of the constriction and the actual separation of the 

 two halves is taken as one half hour. 



The relative distances of the two curves from the base line shows 

 the relative dimensions of length and breadth. The vertical rise of 



