BIGELOW: EARLY ‘DEVELOPMENT OF LEPAS. 83 
(Figs. 9-11). If no firm envelope confined the ovum, interfering with 
change in its form, the long axis of the two-cell stage would be per- 
pendicular to the plane in which the cleavage begins; but the vitelline 
membrane evidently does interfere with extension in a direction per- 
pendicular to that plane. Therefore, as the cleavage progresses and the 
resulting cells become more and more spheroidal (Figs. 10-13), a rota- 
tion of the ovum becomes necessary, for evidently the long axis of the 
two-cell stage must approximately coincide with the long axis of the 
vitelline membrane. An examination of the figures makes it appear 
that, as the forming blastomeres become more spheroidal and conse- 
quently increase the length of the axis of the ovum perpendicular to 
the plane of cleavage, pressure is obliquely applied to the vitelline mem- 
brane with the result that the ovum as a whole rotates, and gradually 
the dividing ovum adjusts itself to the form of the vitelline membrane. 
The cleavage plane becomes transverse or oblique, depending upon the 
amount of rotation necessary to meet adjustment. With a relatively 
wide vitelline membrane the rotation is less than 90°, for the divided 
ovum can then become adjusted to an oblique axis of the membrane, 
and the cleavage plane consequently remains oblique. 
A rotation of the ovum as a result of cleavage has also been shown in 
the case of the rotifer Callidina, described by Zelinka (’91). Like that 
of Lepas, the ovum of Callidina is ellipsoidal and surrounded by a rigid 
membrane. The polar body is situated at one end of the ovum, and 
the cleavage plane passes through this point. Zelinka figures an ob- 
lique spindle, but no sections showing the relations in the various stages 
of mitosis. According to Zelinka the rotation of the ovum occurs after 
division, but the extent of the cleavage plane at the time of rotation 
was not determined by study of sections. It seems probable that, as 
in the cirripede ovum, the rotation may be found to take place during 
the division. 
Jennings (’96, p. 20), commenting upon the rotation in Callidina, 
writes :— “It thus appears that in Callidina the direction of division 
itself is determined neither by the principle of Berthold [surface ten- 
sion] nor that of Hertwig [spindle in long axis of protoplasmic mass], 
but that the later arrangement of the cells might be held to be due to 
the action of Berthold’s principle.” The conditions in Lepas appear to 
be similar to those in Callidina, and Jennings’ conclusion is applicable 
in the case of the cirripede. 
In the eggs of some nematodes there are conditions at the time of 
fertilization very similar to those existing in Lepas. The contiguous 
