Effect of a Centrifugal Force upon Development and Sex 135 



2 If eggs are centrifuged a short time before maturation when 

 the nucleus is intact the nucleus is carried to the top of the clear 

 zone against the bottom of the pink zone. 



3 Very little redistribution of the egg materials takes place 

 before the first cleavage. In consequence the segmented egg retains 

 the distribution of materials impressed on it by the centrifugal 

 force. 



4 The first cleavage plane always appears at that end of the 

 egg at which the pink zone and germinal vesicle are located. It 

 forms across one end as in the normal egg separating a smaller 

 and a larger cell. 



5 The egg centrifuged after the polar spindle has formed 

 shows that the spindle does not move from its original position. 

 Its location determines the position of the first cleavage plane in 

 so far as this appears at the end of the egg nearest to where the 

 spindle lies. 



6 Normal animals, both males and females, develop from 

 centrifuged eggs and these have been reared to sexual maturity. 



7 The sex of animals developing from large (female) or small 

 (male) eggs is not affected by the centrifugal force nor is the sex 

 ratio in the descendants of females developing from centrifuged 

 eggs altered. 



Zoological Laboratory 



Columbia University 



April 25, 1908 



LITERATURE 



Hertwig, O. '97 — Ueber einige am befruchteten Froschei durch Centrifugalkraf 

 hervorgerufeneMechanomorphosen. Sitzungsbere preuss. Akad. 

 Wiss. Berlin, math-phys. Klasse. 

 '99 — Beitrage zur experimentellen Morphologic und Entwicklungs- 

 geschichte. 4. Ueber einige durch Centrifugalkraft in der Ent- 

 wicklung des Froscheies hervorgerufene Veranderungen. Arch, 

 f. mikr. Anat., liii. 

 '04 — ^Wietere Versuche iiber den Einfluss der Centrifugalkraft auf die 

 Entwicklung tierischer Eier. Arch. f. mikr. Anat., Ixiii. 



LiLLiE, F. '06 — Observations and Experiments Concerning the Elementary Phe- 

 nomena of Embryonic Development in Chaetopterus. Journ. 

 Exp. Zool., vol. iii. 



