128 THE AMERICAN ARBACIA 



e. Stratification and Polarity 



The Arbacia eggs fall at random in the centrifuge tubes. There is no 

 orientation as they fall. If there is any polarity in the unfertilized egg, 

 the stratification by centrifugal force bears no relation to it. This was 

 stated by Morgan and Lyon (1907) and proven by Morgan (1909) by 

 comparing the position of the micropyle, which indicates the original 

 axis of the egg, with the stratification ; there was no relation. 



The position of the polar bodies, if given off during centrifugation 

 bears no relation to the stratification but they may be formed in any 

 region, in the pigment zone or yolk or even at the oil cap (Plate II, 

 Photographs 14, 15). 



f. Redistribution of Stratified Material and Return to Spheres 



If the eggs, after centrifuging, are returned to sea water, the stratifica- 

 tion gradually disappears, and the granules are again scattered through- 

 out the egg. The first layer to be lost is the mitochondrial layer; this 

 becomes indistinct almost immediately and cannot be distinguished 

 after 10-15 minutes. The oil cap remains longest, often for 24 hours. 



If the eggs have become dumbbells when the centrifugal force is 

 removed they gradually return to the original spherical shape. Shapiro 

 (1941) has studied the kinetics of the return in sea water, and in sea 

 water without calcium. In sea water they round up, at first rapidly 

 (several minutes), later more slowly. In sea water without calcium they 

 elongate more and contract more quickly. 



Normal eggs, uncentrifuged, when deformed by squeezing through 

 a capillary tube (diameter 50 [x; tgg 74 [ji) round up too quickly to 

 be accurately timed by eye. Moving pictures indicate that the return 

 takes about o.i second (E. N. Harvey and Shapiro, 1941). 



If fertilized, the elongate eggs do not round up, but remain elongate 

 during cleavages and as blastulae (Plate VIII, Photographs 1-8). 



