The finger-like projections of the chorionic membrane that burrow into the endometrium and make up the fetal portion of the placenta are called:

1. The embryonic portion of the placenta develops from the chorionic membrane, formed by the trophoblast and extraembryonic mesoderm. 2. This chorionic membrane produces finger-like structures that penetrate into the endometrium. 3. These structures chorionic villi. 4. They form the fetal portion of the placenta and provide the surface for maternal-fetal exchange, unlike fimbriae, which are unrelated uterine-tube structures. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 28 "Development and Inheritance", p.1332 §Development of the Placenta_