The reptilian clade Squamata is defined primarily by osteological synapomorphies, few of which are entirely unambiguous. Studies of developing squamate eggs have revealed a uniquely specialized feature not known to occur in any other amniotes. This feature—the yolk cleft/isolated yolk mass complex—lines the ventral hemisphere of the egg. During its formation, extraembryonic mesoderm penetrates the yolk and an exocoelom (the yolk cleft [YC]) forms in association with it, cutting off a thin segment of yolk (the “isolated yolk mass” [IYM]) from the main body of the yolk. The YC–IYM complex has been observed and described in more than 65 squamate species in 12 families. In viviparous species, it contributes to the “omphaloplacenta,” a type of yolk sac placenta unique to squamates. The only squamates known to lack the IYM are a few highly placentotrophic skinks with minuscule eggs, viviparous species in which it clearly has been lost. Given its absence in mammals, chelonians, crocodylians, and birds, the YC–IYM complex warrants recognition as a developmental synapomorphy of the squamate clade. As in extant viviparous lizards and snakes, the YC–IYM complex presumably contributed to the placenta of extinct viviparous squamates.

中文翻译：

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鳞状爬行动物的发育突触。

爬行类进化枝的鳞片主要由骨科突触所定义，几乎没有什么是明确的。对鳞状卵发育的研究表明，独特的特殊特征在其他任何羊膜动物中均未见。卵黄裂/分离的卵黄质复合物这一特征使卵的腹半球成直线。在形成过程中，胚外中胚层穿透卵黄，并形成卵囊外胚层（卵黄裂[YC]），从卵黄质的主体切下一小段卵黄（“分离卵黄块” [IYM]）。蛋黄。YC–IYM复合物已经在12个科的65个以上鳞状物种中观察到并描述。在胎生物种中，它会导致“ omphaloplacenta”，这是鳞状上皮细胞特有的一种卵黄囊胎盘。唯一缺乏IYM的鳞状上皮是一些带有极小卵的高度胎盘营养性石龙子，这些卵子显然已经消失了。鉴于它在哺乳动物，凯隆人，鳄鱼和鸟类中均不存在，因此YC–IYM复合物应被确认为鳞状进化枝的发育突触。就像现存的胎生蜥蜴和蛇一样，YC–IYM复合物可能是导致已灭绝的胎生鳞状细胞的胎盘的原因。