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Using time-lapse video recording and comparing successive digital images, we found that 38% of Xenopus laevis embryos (n=118) exhibited rotation during the second cell cycle. This rotation, which we term the second rotation, started approximately during the appearance of the first cleavage furrow and proceeded clockwise or counterclockwise around the vertical axis. Rotations lasted for 5-30 minutes, i.e. up to the beginning of the third cell cycle. The mean rotation angle was 36.4 degrees, with a maximum rotation of 77 degrees. No mortality was observed among the embryos exhibiting rotation. The second rotation was observed to be similar to the well-known fertilization rotation which takes place during the first cell cycle. The possible nature and significance of the second rotation are discussed.
Fig. 1. Rotation in a Xenopus laevis embryo after the appearance of the first cleavage furrow. This rotation (A-D) takes place inside the perivitelline space counterclockwise around the vertical axis, to about 60 ̊, and lasts from the appearance of the first furrow until the appearance of the second furrow. (A) 1 min, (B) 6 min, (C) 12 min and (D) 29 min after the first furrow appearance; the second furrow appeared 1-2 min after the stage shown in (D). The embryo inner jelly layer (JL) is seen on the outside of an embryo (E); the outer jelly layer had been removed with needles. The fertilization envelope (fe) is seen near the upper edge of the furrow (B,C). The very thin perivitelline space is located between an embryo and the fertilization envelope. Bar, 1 mm.
The serial microphotographs (Nos. 1-28)
demonstrate the econd rotationin
one Xenopus laevis embryo. We found
that such rotation, which we termed the
second rotation, took place during the second
cell cycle in about 38% of Xenopus
laevis embryos. It started approximately
during the appearance of the first cleavage
furrow and proceeded clockwise or counterclockwise
around the vertical axis. Rotations
lasted for 5-30 minutes, i.e. up to the
beginning of the third cell cycle. The mean
rotation angle was 36.4 with a maximum
rotation of about 80 The nature of the
second rotation is unknown till now. It may
be a rotation of whole embryo or rotation of
embryo cortex together with developing
first furrow. We suppose that the latter is
more probable. The second rotation which
can be seen in serial images (Nos. 1-27)
proceeds counterclockwise around the vertical
axis. This rotation begins when the first
cleavage furrow arises and lasts for all the
second cell cycle (about 30 minutes), up to
the second cleavage furrow appearance (No.
28). The angle of this rotation is about 74
(Intervals between Nos. 1-24 are about 1
minute and between Nos. 24-28 are 2-3
minutes).
