176 K. S. LASHLEY 



by the simple statistical formulae devised by Pearson and others 

 (Davenport '04; Yule '12). In the computation of the standard 

 deviations for size Sheppard's correction was not used. All calculations 

 were made with the aid of the 'Brunsviga' computing machine, and 

 later repeated de novo. 



Detailed examination of two clones 



On June 10, 1912, twenty-five specimens of Hydra viridis, 

 collected from a limited area in a small undrained pond, were 

 isolated in individual culture dishes. They began to multiply 

 rapidly under the favorable conditions of the cultures. The 

 new-formed buds were each placed in a separate dish, labeled, 

 and the number of their tentacles recorded within 48 hours after 

 they separated from the parents. After a few days it was found 

 impossible to care for all the descendants of the stem mothers and 

 it seemed best to discard the majority of the clones. Two of 

 them, A and D, which seemed somewhat diverse were retained. 

 It soon became necessary again to reduce the number of cultures, 

 as not more than 300 animals could be cared for at one time. 

 An equal number of each of the two clones was therefore selected 

 for high, intermediate, and low numbers of tentacles, and the 

 cultures were so arranged as to give not less than ten progeny 

 from as many parents as possible. Since the selections were 

 approximately equal in both directions from the means of the 

 clones, the average number of tentacles of the two may be used 

 for comparison, although the coefficients of variation may be 

 affected and complications are introduced in the analysis of the 

 data for the problem of inheritance within the clone. 



The breeding was continued until the latter part of August, 

 when the first experiment was brought to a close, and the polyps 

 on hand were placed in mass cultures and left for two weeks. At 

 the end of this time a single polyp was taken from a mass culture 

 of each of the clones and used as the starting point of a subordi- 

 nate clone {A' and D') bred in individual cultures as in the first 

 experiment, except that no Elodea was included in the cultures. 



Differences in number of tentacles and size. The total number 

 of pedigreed descendants of 'stem mother' ^4 obtained before 



