An Observation on Parthenogenesis. 



155 



The dimensions taken were : — 



(1) The total length of the body measured along a line passing ventrally 

 from the base of the spine and cutting the convex surface of the head 

 opposite the middle of the compound eye (AB, see figure). 



(2) The length of Uie protopodite of the 2nd antenna of the right side. 

 The measurement was made on the posterior surface of the proto- 

 podite along a line parallel to the dorsal edge. At the articulation 

 with the head the exo-skeleton of the protopodite possesses a well 

 defined point, which forms a good inner limit to the measurement 

 (CD). 



Since, under favourable conditions, these animals continue to grow 

 throughout life, the second dimension was expressed in terms of the 

 first, thus Length of B protopodite x W0Q 

 lotal length of body 



The mean of the relative length of the protopodite sinks as the 

 animal grows, but between a body length of 2'4 mm. and 3*6 mm. (the 

 total range of size) the change would not be large. I find that at the 

 time of measurement the offspring were constantly somewhat smaller 

 (0'4 to 0*5 mm.) than the parents, but as this applies to all the broods 

 which were measured, the rise in the mean of the offspring would not 

 affect the correlation surface. 



From twenty-three Daphnia (themselves originating by partheno- 

 genesis) broods were produced consisting of three to six individuals. 

 The parents were measured, and the offspring were allowed to grow up. 

 On measuring the offspring it was at once obvious that the children of 

 the same brood exhibited very considerable variability. 



In the following table (p. 156) the results of the measurements are 

 displayed in a correlation surface. 



The table illustrates the variability of children of the same partheno- 

 genetic family, and we can further see, for example, that offspring 

 with a parentage of 169*5 thousandths exhibited a range of variation 

 159-5— 181-5 thousandths. 



The following constants were calculated : — 



