STUDIES WITH THE CENTRIFUGE 37 



of the 16-cell stage several microscopic fields were picked out at 

 random in each watch glass and all the normal and abnormal eggs 

 in each field were counted. Wherever the figures are given for 

 gastrula stages, they are necessarily less accurate because it is 

 difficult to compare the number of swimming gastrulas scattered 

 around the edges of the dish with the dead eggs concentrated at 

 the bottom. However, I counted always an equal number of the 

 most crowded fields, both of dead and living, and in this way 

 endeavored to obtain ratios which could be compared. The error 

 in the individual cases gives too high a per cent of mortality. 



TABLE 1 



I PERCENT 



STAGE AND DURATION OF EXPOSURE IN THE CENTRIFUGE MORTALITY 



16-CELL STAGE 



N6rmal Dead 



Normal control — fertilized immediately after removal 

 from the animal 259 123 I 32 



Centrifuged for 3 minutes— 10-15 minutes after removal 

 from the animal — then fertilized 186 67 26 



Centrifuged for 2 minutes— 20-30 minutes after removal 



from the animal — then fertilized <• . 195 17 8 



Centrifuged for 1 minute— 40-50 minutes after removal 



from the animal — then fertilized 204 69 25 



Centrifuged for 2^ minutes— 45 minutes after fertiliza- 

 tion 



Centrifuged for 2h minutes— 50 minutes after fertiliza- 

 tion 141 26 15 



207 51 19 



The results of the experiments are given in the accompanying 

 tables. All the eggs used in the experiments of table 1 were taken 

 from a single pair of animals. A comparison of the percentages 

 of mortality in the table indicates that at least an hour's exposure 

 in the dishes before the beginning of the experiment does not 

 injure the eggs or sperm, for the mortality is greater in the con- 

 trols than in the eggs which have stood thirty and fifty minutes 

 in the dishes. The table also indicates that there is no greater 

 mortality in eggs, which are centrifuged in the spindle stages pre- 

 ceding the first cleavage, than in those centrifuged before fer- 

 tilization or in the controls. 



