While mesoderm differentiates in the embryo other phenomena happen, namely the formation of extraembryonic membranes and neurulation. The formation of the extraembryonic membranes starts when the egg is descending the oviduct with an initial agglomeration of albumen around the egg forming the chalaza. Subsquently air is entrapped (forming the air sac) forming the inner shell membrane. Another external eggshell membrane involves the previous and finally a porous shell is added by the shell gland from the oviduct, forming a rigid calcareous eggshell.
Around the embryo the three germ layers form important protective membranes involving and surrounding it. An involution of the archenteron will form the allantoic membrane, which is composed of endoderm and splanchnic mesoderm. Overfolding of the ectoderm and somatic mesoderm will form the amnion over the embryo. Expansion of the mesoderm and ectoderm towards the side opposite of the embryo will form the extraembryonic portion of coelom.