250 ROLE OF CYTOPLASM [CH. 



"the visible substances of the egg that can be centrifuged 

 are not organ-forming" (1910, p. 613). In Arbacia the nor- 

 mal axis of the egg is marked by a funnel in the outer mem- 

 brane, and it was found that the stratification produced by 

 centrifuging had no constant relation to the original axis. 



FIG. 31. Segmentation of centrifuged eggs of the Sea-urchin Arbacia, 

 from MORGAN (1909). 



i. Centrifuged egg before segmentation ; the attachment funnel on the left 

 marks the original axis of the egg. 2, 3. First and second cleavages, 

 determined by induced polarity. 4, 5, 6. Fourth cleavage ; the position 

 of the micromeres depends on the original polarity, and not on that 

 induced by centrifuging. 



"The first cleavage begins, almost invariably, at the white 

 cap where the nucleus lies and cuts through the egg, 

 dividing the stratified materials at right angles. The second 

 plane is at right angles to the first, lying near the line of 

 separation of yolk and clear zone, and the third at right 



