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. 每個(gè)胚層產(chǎn)生特殊的胚胎組織。密實(shí)的原腸胚通過(guò)細(xì)胞運(yùn)動(dòng)進(jìn)入內(nèi)胚層和中胚層，爬蟲(chóng)，鳥(niǎo)，哺乳動(dòng)物的胚胎都如此。因此提出這樣的理論，鳥(niǎo)類(lèi)和哺乳動(dòng)物是由爬蟲(chóng)進(jìn)化而來(lái)的。