Fluid-filled central region of a blastocyst.
The cavity inside the blastula. [The_evolution_of_organ_systems_(2007)_Oxford:_Oxford_University_Press, p.150, see_Schmidt-Rhaesa_A][VHOG]
A blastocoel forms during embryogenesis when a zygote (a fertilized ovum) divides into many cells through mitosis. The blastocoel is the cavity of the blastula, a closed space which is invaded by early mesenchyme and later obliterated by the enlarging archenteron.[AAO]
In the development of many animals, a hollow blastula stage is formed during early embryogenesis. Such a hollow blastula is even regarded as an autapomorphy of Metazoa and is present already in some sponges.[well established][VHOG]