Final answer:
The threadlike projections on the chorion are called chorionic villi, which are essential in the development of the placenta.
Step-by-step explanation:
The threadlike projections growing in tufts on the external surface of the chorion are known as chorionic villi. These projections burrow into the endometrium and are key in the development of the placenta, which provides nutrients and oxygen to the developing embryo and removes waste products. The chorionic villi emerge from the chorionic membrane, which is formed from extra-embryonic mesoderm cells and the trophoblast, and they play a crucial role in the formation of the fetal portion of the placenta.