INHERITANCE IN ASEXUAL REPRODUCTION 175 



E.rperimental methods 



Hydra is easily cultivated in the laboratory, provided that a con- 

 stant supply of food is available. The polyps used in the experiments 

 were collected from various sources, usually from ponds containing 

 Nitella or Elodea. In the greater part of the work each polyp was 

 kept separately in a small Stender dish with about 5 cc. of water. It 

 was found necessary to wash these dishes in boiling water every second 

 day in order to keep down bacterial growths and maintain the polyps 

 in good health. A small species of Cyclops was used exclusively as 

 food. Attempts to breed them in sufficient numbers in the laboratory 

 failed and all food material was obtained by towing with a plankton 

 net in an artificial pond. The supply so obtained was further con- 

 centrated so that from one to two hundred Cyclops were given to each 

 Hydra every second day. (A healthy specimen of H. viridis will eat 

 ten or more Cyclops daily and the large number supplied allowed each 

 polyp to capture as many as it could eat). At first, a growing stem of 

 Elodea was kept in the culture with each Hydra but as this was found 

 to be unnecessary and a possible source of contamination it was dis- 

 continued after the first experiment. 



All the cultures were kept at approximately the same level upon a 

 broad table-top where they were exposed to uniform conditions of light 

 and temperature. The cultures of various clones were divided into 

 small groups and the order in which these were fed and arranged upon 

 the table was varied from day to day. While conditions may have 

 varied in the different individual cultures, there seems to be no reason 

 for believing that such differences did not average out in each of the 

 clones, considered as a whole. 



Besides the individual cultures a number of mass cultures of different 

 clones were kept under similar conditions. Numbers of polyps were 

 placed in 6-inch battery jars filled with .water from the food pond. 

 The water in the cultures was renewed weekly, and where, two clones 

 were being bred for comparison the water from the culture jars con- 

 taining the two clones was interchanged frequently. 



The number of tentacles of the polyps was counted under the Zeiss 

 binocular, and in the case of the polyps in individual cultures the 

 number of tentacles of the buds was recorded within 48 hours of the 

 time when they were released from the parents. The numbers of ten- 

 tacles of the parents were recorded also at the time when each bud was 

 produced. Measurements of size were made in the following way: 

 The polyps, contracted, were placed on a grooved slide under a com- 

 pound microscope. When they expanded until the length was about 

 four times the diameter, an outline of the body was made with a camera 

 lucida. The area of this outline was measured with a planimeter and 

 from this, treated as the area of the longitudinal section of an elipsoid, 

 the volume was computed. This method is subject to about 25 per 

 cent error, but is much more accurate than measurements of a single 

 diameter. The constants given in the following pages were computed 



