10 SELECTION IN CLADOCERA ON THE BASIS OF 



cloth on the first straining, they, meantime, having had time to 

 develop before the later straining. The third straining is probably 

 quite unnecessary, but is used as an added precaution. 



The culture-water contains a small amount of the original (now 

 thrice-strained) sediment, has much material in suspension and in 

 solution, and is dull brownish-gray in color. It settles and clears 

 slowly, so that young daphnids placed in the culture-bottles are 

 visible only with difficulty for a day or two. 



The animals when collected were isolated in 200-c. c. wide- 

 mouthed bottles filled to a depth of near 5 cm. (about 100 c. c. in 

 quantity) with culture-water. The young of the first brood from 

 the wild mother were transferred during the first day of life to indi- 

 vidual bottles. A single individual each was placed in the No. 1 and 

 the No. 2 bottles, while three or more were placed in a third bottle 

 designated as a " prime'' bottle to serve as reserve stock. Transfers 

 were made in like manner in subsequent generations, except that 

 Sunday's broods were transferred Monday, when one day old. The 

 bottles of the parent generation were retained as additional reserve 

 stock until a second descendent generation was obtained. The 

 grandmother generation was then discarded, a few individuals being 

 preserved for morphological study if later desired. 1 



For many months after the cultures were begun, and at frequent 

 intervals afterward, large samples of the culture-water, after its 

 three strainings, were set aside in glass jars. Observations were 

 made to determine if daphnids of any sort developed in this water 

 from eggs passing through the straining-cloth or in any way introduced 

 by accident during its handling. None appeared at any time. 



In addition to the three strainings of the culture-water and the 

 general precautions observed in handling the stock, the system of 

 handling the material in itself served to check out any possible con- 

 tamination. The No. 1 and No. 2 bottles each contained a single 

 individual and the prime bottles three or more individuals. Any 

 contamination must have been detected, except in the case of an 

 individual of the same species and very nearly the same age in a No. 

 1 or No. 2 bottle in which the individual belonging in the bottle died 

 within a day after having been placed therein. The likelihood of 

 the coincidence of the introduction of an individual of the same age 

 and of the same species into a bottle in which the rightful individual 

 died within a day is so slight as to arouse little concern. If contami- 

 nation occurred in a prime bottle, to escape ready detection it would 

 have to be an individual of the same species and of very nearly the 

 same age as the individuals belonging in the bottle. Further, the 



1 In case the No. 1 individual is lost, young are transferred from the No. 2 bottle, or if the 

 No. 1 and No. 2 are both lost, the transfers are made from the prime bottle. In case the prime 

 too is lost, extra transfers are made from the first or a later brood of the No. 1 or other surviving 

 individual of the parent generation, and young from one of these transfers are used to start 

 the new generation. 



