84 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 



surfaces of the pronuclei are in a plane which is perpendicular, or slightly 

 oblique, to the long axis of the ellipsoidal egg, and the spindle often 

 begins to form with its long axis in the same transverse plane. Several 

 investigators, among whom may be cited Auerbach ('74, p. 212, Taf. 4) and 

 Ziegler ('95, pp. 379-387), have observed that there occurs a turning 

 of the pronuclei around each other so that their contiguous surfaces and 

 the spindle axis come to coincide with the chief axis of the egg. This 

 turning of the pronuclei and spindle appears to be brought about by 

 streaming movements of the substances of the egg. In addition to these 

 observations on the nuclei during their rotation, there is evidence in the 

 two-cell stage of the nematode that the egg as a whole has not rotated, 

 for the polar cell remains in the long axis of that stage 90 from the 

 equatorial cleavage plane. 



As a result of the turning of the pronuclei and the consequent longi- 

 tudinal position of the spindle, the nematode egg divides in such a plane 

 that the two-cell stage does not require readjustment in order to ac- 

 commodate its long axis to that of the surrounding egg envelope. Thus 

 the turning of the pronuclei and spindle in the nematode eggs affects 

 the orientation of the two-cell stage as completely as does the rotation 

 of the dividing egg as a whole in the case of Lepas. My observation 

 that in L. anatifera the spindle often appears to begin its formation in 

 a transverse plane and then becomes oblique, suggests that there is a 

 tendency towards coincidence of the spindle axis with the long axis 

 of the egg. If such a tendency really exists, it is inhibited by some 

 unknown conditions, possibly the yolk-mass influencing the streaming 

 of the protoplasm, and as a result the cleavage plane is formed in 

 such a position that the two-cell stage must become readjusted to 

 the vitelline membrane. 



Summary of the First Cleavage. 



It has been shown that in L. anatifera, L. fascicularis, and a species 

 of Balanus, the cleavage plane lies at the beginning of cleavage approxi- 

 mately in the long axis of the unsegmented ovum as well as that of the 

 vitelline membrane, and passes through the animal pole. During the 

 division a rotation of the ovum as a whole through an arc of 90 takes 

 place, so that at the close of the division the plane of cleavage coincides 

 with the transverse axis of the vitelline membrane. 



The evidence afforded by preserved material and published figures 

 makes it probable that a rotation of the dividing ovum occurs in all 



