Each layer gives rise to specific tissues during embryonicdevelopment. A variation ingastrulation, involving the movement of cells into endodermal and mesodermalpositions through the thickened primitive streak, arose in reptiles and canstill be seen in bird and mammalian embryos, lending support to the theory thatbirds and mammals evolved from reptiles. 每个胚层产生特殊的胚胎组织。密实的原肠胚通过细胞运动进入内胚层和中胚层，爬虫，鸟，哺乳动物的胚胎都如此。因此提出这样的理论，鸟类和哺乳动物是由爬虫进化而来的。
