30 GEORGINA B. SPOONER 



is the same in all the eggs of the sac; but, owing to the variant 

 direction of the cleavage planes, the layers may be parallel, per- 

 pendicular or oblique to the cleavage (fig. 10). All three conditions 

 may occur in the same egg sac. So far as visible substances are 

 concerned, there is more reason to expect orientation in the two- 

 cell stage than before. On the other hand, if the egg is supposed 

 to orient itself with respect to its original egg axis or polarity, 

 this factor should be as potent after cleavage as before. I think, 

 therefore, it is fair to assume that the eggs do not orient them- 

 selves in the machine. 



In spite of such disturbances of the egg normal embryos regu- 

 larly result and reach the adult stage at the end of three weeks. 

 Egg sacs of animals, which have hatched from centrifuged eggs, 

 have been centrifuged and normal embryos obtained. Moreover, 

 normal embryos result no matter whether the egg is centrifuged 

 while the resting nucleus is intact or after it has broken down and 

 the spindle is formed. The proof of this point was made possible 

 by the fact that the eggs in both sacs of a single animal are in 

 approximately the same stage of development. Their cleavages 

 cut through within three or four minutes of each other. Thus it 

 was possible, by killing one of the sacs immediately after centri- 

 fuging, to determine the stage at which the eggs were centrifuged. 

 After the cleavage of the second sac had been observed the sac 

 was isolated in a watch glass, containing tap water to which a few 

 drops of culture media were added from a jar in which there had 

 been no cyclops. It may be said here that in all isolation of cen- 

 trifuged eggs care was taken to guard against the possible intro- 

 duction of egg sacs or small nauplii from other individuals either 

 by way of pippettes or culture media. 



The spindle after centrifuging 



Preserved and sectioned sacs from four animals contained 

 eggs with metaphase spindles. In the corresponding living sacs 

 the eggs divided perpendicularly to the layers in three to ten 

 minutes after centrifuging and normal embryos hatched in a 



