blood-filled spaces within endometrium, formed following blastocyst implantation and trophoblast erosion of the maternal sinusoids.

Latin for "lakes": one lacuna, two lacunae. Refers particularly to spaces that form within the syncytiotrophoblast (which therefore continues to line the spaces), separating the SCT into strips of land and little peninsulas that will develop into chorionic villi, as the lakes coalesce into one large intervillous space, into which maternal arteries will open, delivering blood to the placenta as it is being formed.

Spaces left among the tissues in some of the lower animals and serving in place of vessels for the circulation of the fluids of the body.