Amniote yolk sacs: diversity in reptiles and a hypothesis on their origin

Oviparous amniotes produce a large yolky egg that gives rise to a free-living hatchling. Structural characteristics and functional attributes of the egg are best known for birds, which have a large mass of fluid yolk surrounded by an extraembryonic yolk sac. Yolk nutrients are delivered to the embryo via the vascular yolk sac. This developmental pattern and nutrient transport mechanism is thought to be representative of all other lineages of amniotes. Recent discovery of a snake with cellularized yolk organized around a meshwork of blood vessels reveals an additional pattern for yolk mobilization, which may also occur in other squamate reptiles (lizards and snakes). This complex yolk sac raises interesting questions about developmental mechanisms and suggests a possible model for the transition between the egg of anamniotes and that of amniotes.