422 L. V. HEILBRUNN 



handle of the centrifuge, usually two or three seconds elapsed 

 before the first indication of zones appeared. In one case four 

 seconds were required, but this was exceptional. In another 

 case a single turn was sufficient to show zones, but these eggs of 

 unusually low viscosity were not absolutely normal and at seg- 

 mentation they showed poorly indented cleavage furrows. Ap- 

 parently the viscosity varies with the temperature, decreasing 

 slightly as the temperature rises from 19° to 25°. More data 

 are necessary, however, in order to establish this point. 



The viscosity changes during mitosis are shown in the follow- 

 ing tables. Eggs were centrifuged at rapid intervals between 

 fertilization and cleavage. One series of tests was made in 

 which the centrifuge handle was turned three times in three 

 seconds, another series in which it was turned four times in 

 four seconds, and so on. In all of the tests recorded here, a 

 uniform speed of one turn per second was maintained. This 

 was continued for from two to twenty seconds in the various 

 tests. Because of the difficulty in starting, it is not easy to 

 turn the handle twice in exactly two seconds. After a turn or 

 two has been made, however, it is a simple matter to keep the 

 speed uniform. 



In the tables the first column shows the number of seconds 

 elapsed after fertilization at the time a given test was made. 

 In working with various sets of eggs on different days, the length 

 of time between fertilization and cleavage varies considerably. 

 In order to compare one set of eggs with another, it is necessary 

 to reduce all the time records to some standard basis. The 

 average time between fertilization and cleavage is about fifty 

 minutes. Fifty minutes was therefore taken as a standard time 

 interval, and by a simple calculation it was determined in each 

 case how much time would have elapsed if the total time until 

 cleavage had been fifty minutes. These figures are given in 

 the second column and are referred to as 'standard time.' There 

 is always some variation in the time of cleavage of individual 

 eggs of any one batch. The time of first cleavage was taken 

 as the time when 50 per cent of the eggs had completely divided. 

 In order to obtain this time, rapid counts had to be made at 

 frequent intervals. These counts are recorded in the third 



