Blastoderm

BLASTODERM (from Greek blastos-germ and derma-skin), the rudimentary or germinal skin (synonyms: embryonic plate, germinal disc), a name given by Kölliker to the cellular layer that arises from the cleavage of a chicken egg at the site of the so-called blastodisc. In a laid egg, the blastoderm consists of two layers; the inner one is thickened at the edge and forms a rudimentary ridge (bourrelet blastodermique), by means of which the blastoderm grows. In the center of the blastoderm, the embryonic field (see Area embryonalis) forms and the embryo body develops. By the end of the 6th day of incubation, the blastoderm completely covers the yolk, forming

The blastoderm of a chicken embryo (after Hertwig).

a vesicle (vesicula blastodermica). The blastoderm is found, besides in birds, in all animals with partial cleavage (meroblastic eggs): in fish, selachians, reptiles; among invertebrates-in spiders, crustaceans, cephalopods. In mammals, after cleavage is completed, a vesicle filled with fluid forms, in one place of which the embryonic node is located; this vesicle (trophoblast) corresponds to the vesicula blastodermica of birds. The similarity is explained by the fact that the ancestors of mammals had a large amount of yolk and underwent partial cleavage.