The inner layer of the trophoblast, interior to the syncytiotrophoblast in an embryo. It serves to anchor the embryonic chorion to the maternal endometrium. Cytotrophoblasts are stem cells in the chorionic villi. During differentiation, mononuclear cytotrophoblast fuse together into the multinucleated syncytiotrophoblasts. The primary villi has only the cytotrophoblast as an axe. [WP,unvetted]. [ http://placentation.ucsd.edu/glossfs.html http://en.wikipedia.org/wiki/Cytotrophoblast ]