166 K. S. LASHLEY 



If SO, how is it produced and what is its significance? Is there 

 an inheritance of individual differences within the clone? What 

 is the origin of the diversities between races? 



II. VARIATION IN HYDRA 



The only variable character in Hydra considered in detail 

 by previous workers has been the number of tentacles. At the 

 beginning of the present experiments I examined a number of 

 polyps in the hope of finding other variable characters suitable 

 for genetic study. Size, body-form, color, relative length of 

 the tentacles, reaction to mechanical stimuli, distribution of the 

 nettle cells, diameter of the mature nematocysts, and reaction 

 to light were compared. Of these only size seemed at all suitable 

 for statistical study. Variations in practically all the characters 

 of Hydra appear, not only between different individuals, but in 

 the same individual at different times. Hase has shown that 

 the number of tentacles, alone, varies too much in the individual 

 to form a character suitable for statistical treatment ' and Plate 

 ('13) has stated the necessity for the use of the number of ten- 

 tacles at some definite stage in the development of the polyps 

 in future studies of heredity. The same individual variability 

 appears in the size of the polpys so that the size at some definite 

 stage of development must be used in comparative studies. 



The time when the bud separates from the parent marks the 

 occurrence of certain definite physiological processes in the bud 

 and, if the number of tentacles of the bud is taken at this time, 

 it serves as an index of the state of development of the bud at 

 the time when these processes are initiated. This number may 

 be compared, in tests for heredity, either with the number borne 

 by the parent at the corresponding age, or with the number which 

 it bears when the given bud is produced. It is more difficult 

 to find a stage where the size of different polyps is comparable. 

 Growth is very rapid for the first few days after the buds are 

 released and it is not always possible to take measurements of the 

 new buds, as such measurements requne a great deal of time. 

 After four or five days for H. viridis, when the buds have matured 



