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. This arrangement provides a means by which yolk material becomes cellularized, digested, and transported for embryonic use. Ultrastructural observations on late‐stage Alligator mississippiensis eggs reveal elongated, vascular strands of endodermal cells within the yolk sac cavity. The strands of cells are intermixed with free yolk spheres and clumps of yolk‐filled endodermal cells, features that reflect early phases in the yolk‐processing pattern. These observations indicate that yolk processing in Alligator is more like the pattern of other reptiles than that of birds. Yolk processing in embryos of the American alligator is accomplished through formation of elongated, vascular strands of endodermal cells that fill the yolk sac cavity. This pattern is very different from that of birds, but shows similarities to those of other reptiles.