Final answer:
The ejaculatory ducts are the structures that form when the seminal vesicles join the vas deferens and then connect to the urethra.
Step-by-step explanation:
The structure that begins where the seminal vesicles join the vas deferens and then joins the urethra is known as the ejaculatory duct. The ejaculatory ducts are formed by the union of the vas deferens with the seminal vesicles. They play a crucial role in the male reproductive system by carrying sperm from the vas deferens, combined with secretions from both the seminal vesicles and the prostate gland, to form semen.
This fluid provides a medium for sperm to "swim" and the secretions control its pH and provide nutrients to the sperm, which are important for fertilization.