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Abstract
In fertilized eggs of the frog Xenopus, the vegetal yolk mass rotates away from the
future dorsal side (J. P. Vincent and J. Gerhart, 1987, Dev. Biol. 123, 526-539),
and a major rearrangement of the deep animal hemisphere cytoplasm produces a characteristic
swirl in the prospective dorsal side (M. V. Danilchik and J. M. Denegre, 1991, Development
111, 845-856). The relationship between this swirl and determination of the dorsal-ventral
axis was further investigated by attempting to experimentally separate the positions
of the swirl and the dorsal-ventral axis. Eggs were obliquely oriented in the gravity
field to respecify the direction of yolk mass rotation and the position of the dorsal-ventral
axis. When yolk mass rotation occurred in the absence of a sperm, as in activated
eggs, a swirl pattern formed on the side away from which the yolk mass had rotated.
In fertilized eggs tipped with the sperm entry point (SEP) down or to the side, swirl
patterns were always found to form on the side away from which the yolk mass was displaced.
However, in eggs tipped SEP up, in which the yolk mass was forced to rotate away from
the SEP, more complicated rearrangements were observed in addition to the rotation-oriented
swirl. Because the direction of yolk mass rotation was found to be influenced by both
gravity and the actual position of the SEP in obliquely oriented eggs (SEP to the
side), such complicated rearrangement patterns may result from opposing forces generated
by both yolk mass rotation and the expanding sperm aster. Thus, except in cases in
which the influences of SEP position and unit gravity opposed each other, it was not
possible to experimentally separate the position of the deep cytoplasmic swirl from
the direction of yolk mass rotation, and therefore the position of the prospective
dorsal side.