Hippomanes
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Hippomanes (Greek ἱππομανές) are brownish or olive-greenish, elastic formations that arise physiologically in female bovines and equines during pregnancy.
The up to fist-sized hippomanes are either attached to the allantois membrane or swim freely in the allantoic fluid. Occasionally such formations also occur in ruminants (cows, sheep, goat) and pigs.[1] Similar formations have also been found in sea-cow, lemur and hippopotamus, but the objects found in the amnion and allantois are distinguishable.[1] This has caused the confusion over the word "hippomanes" in scientific literature, with propositions to use it only for objects found in Equidae pregnancies.[1]
Hippomanes arise from unconsumed and thickened nutrient fluid (histiotrophe) of the placenta, in concentric layers around the centre of allantoic calculi of tissue debris.[1] The chemical composition is different from amniotic fluid, which along with the insolubility suggests that it's not a result of a simple precipitation process, but a denatured mucoprotein complex.[2]
Hippomanes first appear in pregnant horses and zebras ca. 85 days from conception, earlier there are small white flecks of tissue debris instead.[1] The changes in hippomanes' color correspond to changes in color of allantoic fluid.[1] They are often surrounded by shed cells.