STUDIES WITH THE CENTRIFUGE 29 



rounding protoplasm. Gradually the green spheres move along 

 the periphery, forming two points on opposite sides of the egg, 

 through which the first cleavage plane always cuts. During the 

 same interval the yolk spheres along the protoplasmic border 

 show a vibratory sort of movement and slowly encroach on the 

 clear band, the movement into the protoplasm being most marked 

 opposite the points of the green cap. After ten to fifteen minutes 

 the clear band is quite blurred so that the nucleus or spindle, as 

 the case may be, is roughly outlined as in the normal egg and on 

 the yolk side of the aster the radial arrangement of the yolk 

 spheres is often quite clearly seen. The first cleavage is alwaj^s 

 perpendicular to the layers so that even in eggs which have been 

 centrifuged in the anaphase or telophase the process of remixing 

 goes on unhindered for another half hour, or in many cases an 

 hour, till the first parallel cleavage occurs. That the tendency 

 of the centrifugal layers to remix is not responsible for the nor- 

 mal development after centrif uging is shown by an experiment in 

 which the unsegmented eggs were placed in the water centrifuge 

 and revolved at a moderate rate of speed for 3 hours. The sep- 

 aration of materials was thus maintained until after the third, 

 or equatorial cleavage had come in. Thus four cells contained 

 the yolk spheres and the other four cells contained protoplasm 

 and oil. Normal embryos were obtained from such eggs. There- 

 fore, the separation of the visible substances does not in this case 

 lead to abnormality. 



The direction of the layers is the same in all the eggs of a given 

 egg sac. The sacs are free to swing in any direction, being attached 

 to the animal only at one point. Nevertheless the direction of 

 stratification with reference to the axis of the sac is rarely the 

 same in the two egg sacs of a given individual. The egg sac as a 

 whole, therefore, does not orient itself to the direction of the 

 centrifugal force. The question of the orientation of the indi- 

 vidual egg is more difficult to determine and could not be proved 

 from the unsegmented egg. It is possible, however, to show that 

 immediately after the first cleavage, the eggs do not orient them- 

 selves. For, if such eggs are centrifuged the direction of the layers 



