Study of 900 Embryos Finds 'Visceral Pillar' Drives Snake Coiling

Q: Why do a rare few snake embryos defy this biological rule and coil counterclockwise despite lacking muscles?

The most likely explanation is that those rare counterclockwise embryos started out with an unusual orientation inside the egg, not that they actively twisted themselves. The study found early coiling happens before muscles exist, so the embryo cannot choose a direction. Instead, the growing body buckles around a slow-growing “pillar of gut” that is tethered to the yolk. Because the yolk is normally fixed on the embryo’s left side, that mechanical setup almost always produces a right-handed spiral. For the small minority that coil the other way, the researchers infer that the embryo was probably positioned differently relative to the yolk from the outset. In that reversed setup, the same passive forces would push the body into a left-handed, or counterclockwise, spiral. The team points to similar left-right reversals seen in a small fraction of bird embryos as a plausible parallel. So the exception does not appear to break the rule so much as reveal it: even without muscles, a rare embryo can coil counterclockwise if its initial placement inside the egg flips the usual geometry between body, gut tether, and left-sided yolk. The researchers stress that this remains an inference rather than a directly tested mechanism, because experimental manipulation of snake eggs is difficult.

3 articles · Updated · IFLScience · Sep 1

A previously unknown "visceral pillar"—a detached gut section and blood vessels—was identified as the structure that makes snake embryos spiral inside the egg, according to a Current Biology study led by Tetsuto Miyashita.
Data from 900 embryos across 39 species showed the gut grows more slowly than the spine, creating a tether along the body axis that forces the rapidly elongating embryo to coil.
The mechanism also explains the usual right-handed curl: with the yolk on the left, the tether pulls to the right early in development before embryos later reposition themselves with muscle control.
Miyashita said the finding is striking because such a severe organ-growth mismatch would usually be lethal, yet snakes appear to exploit it to build their elongated body plan.
The work, begun during COVID-19 lockdowns, relied on counting and measuring museum and lab specimens rather than advanced genetics, underscoring how basic observation can still uncover major developmental mechanisms.