Hi -
There is not yet a definitive answer to your question. Numerous
experiments have been done which have shown conflicting effects of
microgravity on the development of various embryos. The Soviets did a
fair amount of experimenting on this topic, although I cannot find
references for much of it. The most consistently reported alterations
(when alterations are seen) are in the development of the inner ear
(balance and gravity sensing portion) or it's associated brain regions
and bone formation. Major malformations have not been reported by any
groups and failures to develop entirely have been chalked up to
experimental problems rather than due to effects of microgravity. So,
microgravity does not appear to have major teratogenic effects,
although it may affect inner ear development (although NASA
experiments suggest that, at least in chickens, that these changes are
transient).
In 1990 and 1992 there were experiments performed on Mir on quail eggs
which revealed that development and hatching were possible in a
weightless environment. Abnormalities were detected in the areas of
the brain which receive input from the inner ear (which controls
balance) as well as decreases in the amount of bone calcium and
alterations in the microvascular (small blood vessels) composition of
the chorioallantoic membrane (sort of like a placenta for a shelled
critter). Possible confounding factors were thought to be the
stresses of launch upon the eggs. A page which briefly discusses this
and gives references to the publications from the work:
http://spaceflight.nasa.gov/history/shuttle-mir/science/fb/sc-fb-quail.htm
Similar effects were noted in newts: (page with many pictures of the
experiments, the newts, the eggs, etc)
http://spaceboy.nasda.go.jp/lib/shuttle/iml/e/iml_2_02_e.html
NASA has flown chicken eggs and has not seen developmental
abnormalities (even in the bones or inner ears. This page mentions
these results in the context of a large report examining which animal
species to use to further study the effects of microgravity:
http://fundamentalbiology.arc.nasa.gov/EP/reports/NASA_DB_Rev._Panel_Report.doc
The ESA has flown Sea Urchin eggs and has not seen any effect of
microgravity on the development:
http://esapub.esrin.esa.it/sp/sp1206/marth.htm
The medaka fish has been choosen for further experiments upon
development due to it's transparent body (makes detecting anatomical
abnormalities easy), quick replication time, and success in early
experiments (which do not appear to show birth defects).
http://130.69.100.13/SPACEMEDAKA/IML2/e/text/textchap1/chapter1_section6_E.html
A Japanese group using a device to simulate microgravity on the ground
has found that fertilization and early development (up to 96 hours)
are not effected in mice:
http://www.fertilitycommunity.com/bulletins/dec15-2000.html
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