Xenopus laevis embryos can establish their spatial bilateral symmetrical body pattern without gravity.
One assumes that gravity cooperates with the sperm in the establishment of bilateral symmetry in the embryo, particularly in species with yolky eggs.  However, only experiments under genuine microgravity can prove this.  May 2nd 1988 on the TEXUS-17 Sounding Rocket, eggs of Xenopus laevis became the first vertebrate eggs ever successfully fertilized in Space. Fertilization was done in fully automated hardware; the experiment was successfully repeated and extended in 1989.  Here we report a "Space First" from the IML-1 Space Shuttle mission (January 1992):  In similar hardware and under microgravity, artificially fertilized Xenopus eggs started embryonic development.  Histological fixation was pre-programmed at the time gastrulation would occur on Earth and indeed, gastrulae were fixed.  Thus after fertilization in near weightlessness Xenopus embryos do develop bilaterally symmetrically, very probably cued by the sperm alone.
Ubbels
G A
GA
Hubrecht Laboratory, Netherlands Institute for Developmental Biology, Utrecht.
Reijnen
M
M
Meijerink
J
J
Narraway
J
J
D000818:Animals; D016764:Cell Polarity / Q000502:physiology; D004625:Embryo, Nonmammalian / Q000166:cytology / Q000502:physiology; D005260:Female; D005306:Fertilization / Q000502:physiology; D005775:Gastrula / Q000166:cytology* / Q000502:physiology; D008297:Male; D010063:Ovum / Q000502:physiology; D013026:Space Flight; D013084:Sperm-Ovum Interactions; D014893:Weightlessness; D014982:Xenopus laevis / Q000196:embryology* / Q000502:physiology