Which of the following are components of the definitive chorion?: (A) Extraembryonic somatic mesoderm and epiblast, (B) Extraembryonic somatic mesoderm and cytotrophoblast, (C) Extraembryonic somatic mesoderm and syncytiotrophoblast, (D) Extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast
Answer is D. The definitive chorion consists of three components: extraembryonic somatic mesoderm, cytotrophoblast, and syncytiotrophoblast. The chorion defines the chorionic cavity in which the embryoblast is suspended and is vital in the formation of the placenta.