134 David Day Whitney 



in the germinal vesicle stage the later histories of the germinal 

 vesicles differ. In the egg of Chaetopterus the maturation spindle 

 which develops from the germinal vesicle, according to Lillie, 

 migrates to the animal pole if it does not happen to be located at 

 that pole, while in Hydatina the maturation spindle never migrates 

 from the end of the egg into which the germinal vesicle is driven 

 by the centrifugal force. In the egg of Arbacia the female pro- 

 nucleus may be so oriented by centrifugal force that the direction 

 of the first cleavage plane is due rather to its location and follows 

 in consequence the stratification of the materials in the egg. 



None of the previous workers have reared the embryos from 

 centrifuged eggs to maturity because the forms upon which they 

 worked were not suitable for such experiments but Hydatina is an 

 exceptionally favorable form for such work. 



Eggs were centrifuged in various stages of maturation so that 

 the zones of egg materials were differently arranged in their relation 

 to the first cleavage plane, thus making it possible that in some 

 cases the pink material of the egg would be included in the cells 

 that make up the anterior end of the embryo while the gray 

 material would be included in the cells of the foot region or vice- 

 versa. In other cases the material would be more or less equally 

 distributed in the anterior and posterior regions of the embryo. 



In no case was the sex of the eggs changed and such eggs pro- 

 duced a very high percentage of normal young males and females. 

 Furthermore the young females grew to adult animals and pro- 

 duced normal offspring of which the sex ratio was apparently 

 normal. 



It would, therefore, seem that the effect of centrifugal force 

 upon the eggs of Hydatina senta is not sufficient to cause any 

 noticeable change of structure or of sex in the animals that develop 

 from them. 



VI SUMMARY 



I When the unsegmented eggs of Hydatina senta are centri- 

 fuged twenty thousand revolutions the materials in the eggs are 

 stratified into a pink zone, a clear middle zone and a gray zone. 



