The trabeculae become orientated radially and are penetrated by cytotrophoblast cells. They now consist of syncytiotrophoblast at the outside, and cytotrophoblast at the inside. The primary villi sprout from these trabeculae and consist of a core of cytotrophoblast cells surrounded by a syncytial layer. They elongate greatly and penetrate into the lacunae. The system of lacunae is now known as the intervillous space. The trabeculae develop further into stem villi. The layer consisting of both trophoblast and extra-embryonic mesoderm is called the chorionic plate.