326 HOMER W. SMITH AND G. II. A. CLOWES. 



ods of fixation stop all developmental processes at once and the 

 cleavage planes remain clearly visible for many hours. Careful 

 counts were then made on each sample, noting the number of 

 eggs which were undivided and the number which were in each 

 stage of division. By multiplying the number of two-cell eggs by 

 one, the 4-cell eggs by two, the 8-cell eggs by 3, etc., and dividing 

 the total number of divisions by the total number of eggs, the 

 number of divisions per egg in each sample was determined. 

 This figure is an arithmetic index of the degree of development, 

 or if expressed in terms of time, of the velocity of development. 

 By counting two to 3 hundred eggs in each sample, considerable 

 accuracy can be obtained. 



A particular culture of eggs will develop under constant con- 

 ditions in sea water with a mean velocity that remains practically 

 constant so long as the number of cleavages can be accurately 

 counted. Certain individuals will be slower than the mean and 

 others will be faster than the mean, expressing differences in 

 viability or developmental capacity. Such differences may be 

 interpreted from a statistical point of view to indicate the fluctua- 

 tions which any individual may undergo, and the mean to repre- 

 sent the behavior of the average individual. The variations ob- 

 served in the development of different cultures present many inter- 

 esting features which we cannot discuss at this time. It should be 

 pointed out, however, that for studies of developmental velocity 

 under normal and abnormal conditions, the ideal condition is to 

 have a maximum distribution of variants (" slow " eggs and 

 " fast" eggs) so that development will progress over short inter- 

 vals of time (i.e., 15 to 20 minutes) in a uniform manner. 

 Though this condition usually obtains, there are times in the sea- 

 son when the eggs are in such uniform physiological condition that 

 they divide almost simultaneously. At such times the number of 

 divisions per egg increases by abrupt steps. This circumstance 

 can be alleviated by averaging two successive observations on each 

 culture. For the present purposes it will suffice to consider the 

 mean development during the entire period of observation. 



The influence of reaction on the early developmental rate of 

 normally fertilized Aster ms and Arbacla eggs is shown in Fig. I. 

 The data summarized in this figure are taken from several experi- 



