How do Crocodylian embryos process yolk? Morphological evidence from the American alligator, Alligator mississippiensis.

Recent studies have demonstrated a mechanism of embryonic yolk processing in lizards, snakes and turtles that differs markedly from that of birds. In the avian pattern, cells that line the inside of the yolk sac take up products of yolk digestion and deliver nutrients into the vitelline circulation. In contrast, in squamates and turtles, proliferating endodermal cells invade and fill the yolk sac cavity, forming elongated strands of yolk-filled cells that surround small blood vessels.

Ultrastructural analysis of the yolk processing pattern in embryonic pond slider turtles (Trachemys scripta: Emydidae).

Evolution of the large-yolked, amniotic egg required mechanisms by which extracellular yolk could be made available for embryonic development. In birds, the endodermal lining of the yolk sac absorbs and digests the yolk. In contrast, recent studies on lizards and snakes (squamates) have revealed that yolk is processed by means of a proliferating mass of "spaghetti-like" strands formed by endodermal cells attached to anastomosing blood vessels.

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.