Meeting Abstract
Reptiles and birds are widely believed to share patterns of early development, including mechanisms of yolk cleavage and yolk cellularization. These patterns are widely assumed to have been retained from the common amniote ancestor. However, our observations on the lizard Sceloporus undulatus (Phrynosomatidae) have revealed a pattern of yolk processing that is very different from that of birds. Light microscopy and SEM show that as the endodermal cells proliferate, they form clumps of interconnected cells that are filled with yolk droplets. During vascularization, capillaries become encased in these cells, forming masses of elongated, spaghetti-like strands. As a result, the blood vessels are optimally -positioned to transport products of yolk digestion to the developing embryo. Recent studies in our lab have found that this unexpected pattern also occurs in snakes; therefore, it may be ancestral for squamate reptiles. The diversity of yolk processing mechanisms in sauropsids has significant implications for our understanding of the evolution of the amniote egg.
