How snake embryos coil.

Snake embryos are often tightly coiled, but the biological basis for this developmental trait remains a puzzle. To understand how and why they coil, we first investigated directions of coiling within and across 39 snake and other limbless squamate species. In early developmental stages, coiling orientation appears to be nearly fixed right-handed across multiple species. We present evidence that accelerated somitogenesis outpaces gastrointestinal extension at these stages, causing the axial column and somitic series to coil around the delayed visceral tissues. To allow this, the body axis and the gut are physically separate, where the latter forms a pillar-like structure from the stomach to the cloaca. The initial dextral directional bias in these early embryos is likely passively determined by the left-sided yolk mass. Coiling becomes looser and more inconsistent later in development when the embryos acquire mobility through maturation of axial musculoskeletal tissues and with a sufficiently extended gut. These consilient lines of observations reveal, in embryonic coiling, a series of responses of the growing tissues to spatial limitations during snake embryogenesis. As these parameters conflict at some developmental stages (e.g., the offset growth curves between the gut and the body axis), snakes apparently solve some of the many challenges to patterning their startlingly elongate bodies by coiling, which takes advantage of what could otherwise be discordant features of embryonic growth.