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 spomalal 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 
