CENTRIFUGAL FORCE ON EGGS OF CREPIDULA 321 



body the identification of the animal pole in centrifuged eggs is 

 more or less a matter of conjecture, dependent largely upon the 

 position of the egg nucleus or spindle and upon the subsequent 

 movements of the egg substances. After the first polar body 

 has been formed normally the position of the animal pole is no 

 longer uncertain. However, as we have seen in figures 9 to 12, 

 it is possible to move the first polar spindle before it becomes 

 firmly attached to the periphery of the egg, and if such eggs are 

 kept whirling for a long time the polar body may form at other 

 points on the egg than the original animal pole, as shown in 

 figures 36 to 38. 



2. Results of centrifuging during the second maturation division 



(figs. 19-58) 



In the late anaphase of the first maturation division the 

 chromosomes in the egg can not be centrifuged away from the 

 animal pole to which they are bound probably by the interzonal 

 fibers (figs. 13 to 18). When the second maturation spindle has 

 reached its early metaphase this attachment is relaxed, for if eggs 

 are centrifuged at this stage the spindle is carried away from 

 the animal pole along with the cytoplasm and may be trans- 

 ported to any part of the cell (figs. 19 to 21). However, if the 

 second maturation spindle has reached its anaphase at the 

 time the centrifuging begins it can not be moved very much since 

 one pole of the spindle becomes more or less closely attached to 

 the surface of the egg by its astral fibers. In some cases such a 

 spindle may be pulled away from the surface for a short dis- 

 tance without breaking this attachment (figs. 23, 24), but in 

 other cases it remains closely attached to the surface at the 

 animal pole (figs. 22, 25, 29). This variation is probably due 

 to slight differences among various eggs in the age of the spindle 

 and in the firmness of its attachment to the surface, as well as 

 to differences in the strength of centrifuging. 



If the young maturation spindle is centrifuged away from the 

 animal pole (figs. 19 to 21) it takes a position in the cytoplasm 

 between the yolk on one side and the oil cap on the other. It 

 may be carried all the way through the egg from the animal to 



THE JODHNAL OF EXPERIMENTAL ZOOLOGY, VOL. 22, NO. 2 



