A PHYSIOLOGICAL CHARACTER. 11 



chances would be 3 to 1 (since there are three or more individuals 

 belonging in the prime bottle) against young from the strange indi- 

 vidual being used in propagating the line in the unlikely contingency 

 of this bottle being used to continue the line. These facts, combined 

 with the fact that the prime bottles were used only occasionally 

 (when the No. 1 and No. 2 individuals had died) in propagating the 

 strains, make the chances greatly against the perpetuation of a con- 

 tamination of a pure lime, if such a contamination ever did occur. 

 With a single exception, 1 which was readily detected, we have every 

 reason to believe that our selection cultures did not at any time be- 

 come contaminated by the introduction of daphnids in the culture- 

 water or by any other means. 2 



PEDIGREE RECORDS. 



All the descendants of each of the "wild" individuals originally 

 taken into the laboratory are referred to as belonging to a single line, 

 while any subdivisions of the stock within this line are designated as 

 strains. Thus, of each of the lines in which selection on the basis 

 of light reactions was made, there were plus and minus strains, while 

 of certain lines there were several additional strains, all of the strains 

 of any line of course having descended in the laboratory from a single 

 "wild" individual. 



In the records each line is referred to by a certain arbitrary 

 number. The generation of its descent since having been brought 

 into the laboratory is indicated by a letter followed by a subscript. 

 The plus and minus strains are indicated by the plus or minus signs. 

 Thus 695 DH indicates reference to the 264th generation (the letters 

 of the alphabet having been used ten times over and to the letter 

 D on the eleventh time) of the minus strain of Line 695. 3 



Notes were made on loose-leaf sheets, a sheet being used for the 

 notes for the three bottles (No. 1, No. 2, and "prime") of each gener- 

 ation of each strain. Records on this sheet include the pedigree 

 designation, the pedigree of the mother from which the young were 

 derived, and the date of birth of the young, all made at the time of 

 making the transfer. Later, the date of occurrence and (usually) 



1 In one case, early in the course of these experiments, a strange daphnid did appear in 

 one of the culture-bottles. It was a small, rapidly reproducing form, the eggs of which might 

 conceivably be pressed through the straining-cloth, young developed to maturity, and eggs of 

 the next generation produced before the second straining. It was readily recognized as an inter- 

 loper. Such an appearance, even had it been that of a stray individual of the species under cul- 

 tivation, would have been readily detected and leads to no great concern as to the purity of our 

 cultures. 



2 In a few instances two individuals were found in a No. 1 or a No. 2 bottle, due to the 

 lifting out in the pipette of two (instead of one) individuals from the mother's bottle or from 

 the expe- imental tank when making the transfers. The prompt finding of the extra individual 

 added to our confidence in the purity of our cultures, but in every case the bottle in which an 

 extra individual was found was discarded. Since in these cases the extra individual was not of 

 another strain, but merely an extra individual of the same strain, its presence in the bottle did 

 not represent a threatened contamination. 



3 It is to be noted that the minus strains are strains selected for reduced reactiveness to 

 light. No negatively reacting strains occur. 



