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.

中文翻译：

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鳄鱼胚胎如何加工蛋黄？来自美洲短吻鳄，密西西比短吻鳄的形态学证据

最近的研究表明，蜥蜴、蛇和海龟的胚胎蛋黄加工机制与鸟类明显不同。在鸟类模式中，排列在卵黄囊内部的细胞吸收卵黄消化产物并将营养物质输送到卵黄循环中。相比之下，在有鳞类和海龟中，增殖的内胚层细胞侵入并填充卵黄囊腔，形成围绕小血管的细长的卵黄填充细胞链。这种排列方式提供了一种使蛋黄材料细胞化、消化和运输以供胚胎使用的方法。对晚期密西西比鳄鱼卵的超微结构观察揭示了卵黄囊腔内内胚层细胞的细长血管链。细胞串与游离的卵黄球和充满卵黄的内胚层细胞团混合，这些特征反映了卵黄加工模式的早期阶段。这些观察表明，鳄鱼的蛋黄加工更像是其他爬行动物的模式，而不是鸟类的模式。