414 JENNINGS— HEREDITY IN PROTOZOA. [April 24, 



mine as accurately as possible what this part is by a systematic study 

 of growth. 



Material and Methods of Work. 



In order to exclude possible differences due to different ancestry, the 

 study of growth was made with the progeny of a single individual for each 

 of the two groups. Of the caudatitm group a single individual D was iso- 

 lated April 12. This individual was a large one, measuring approximately 

 250 microns in length. From it many cultures were made under various 

 conditions, and all the results on growth in this group were reached with 

 progeny of this individual D, save in cases where the contrary is expressly 

 stated. In the same way the results for the aiirelia group were reached with 

 the progeny of a single individual c, unless otherwise noted. 



The method of work in the statistical study of growth was as follows : 

 Numbers of dividing Paramecia of known descent were isolated and kept 

 for varying periods, so that the age of the individuals was known to within 

 a few minutes or even less. The individuals were then killed at different 

 ages by the use of Worcester's fluid, and measured. In this way the usual 

 size at various ages was determined, and those variations in size that are 

 due only to varying age of the individuals were excluded. By pursuing this 

 method, an approximate curve of growth is obtained and the part played by 

 growth in the observed variations elucidated; much light is in this way cast 

 on many obscure matters. 



To persons who have worked with Paramecium it is unnecessary to point 

 out the extremely laborious and time-consuming character of the operations 

 required. Dividing specimens must be sought for with the microscope, 

 among hundreds of their rapidly moving fellows ; they must be taken up 

 with the capillary tube, isolated, placed in culture fluid, and the time of 

 capture noted. They must then, after lapse of the proper interval, be killed 

 and measured; this is the smallest part of the work. To thus deal with 

 individuals of known age by the hundred involves an incredible amount of 

 •exhausting labor, so that if the mathematical student finds in any stage the 

 numbers employed not always as large as would be ideally desirable, he 

 will realize that there is good reason for this. But it is hoped that the 

 numbers used are amply sufficient, on the whole, for the purposes designed; 

 the results are drawn from the measurement of over 1,500 specimens of 

 known age; together with control cultures of mixed ages in still larger 

 number. 



Especially in the study of individuals that are very young (up to the 

 age of half an hour or so), there is very great difficulty in dealing with 

 large numbers owing to the fact that the time required for picking them 

 out is very large in proportion to the amount of time they are to be kept, 

 so that but few can be dealt with at once. Another great difficulty lies in 

 the fact that to be strictly comparable, the sets of different ages must be 

 chosen on the same day from the same culture; otherwise differences due 

 to cultural conditions show themselves, confusing our results. No culture 

 remains the same for two successive days, and the differences quickly show 



