Final answer:
The duct behind the testis where sperm passes to the vas deferens is the epididymis, where sperm mature before being transported via the vas deferens to form the ejaculatory duct.
Step-by-step explanation:
The highly convoluted duct behind the testis, along which sperm passes to the vas deferens, is called the epididymis. After developing flagella, nearly mature sperm leave the testes and enter the epididymis, which is situated along the top and posterior portion of the testes and is responsible for sperm maturation. The sperm then travel from the epididymis into the vas deferens. The vas deferens transports the sperm behind the bladder and together with the duct from the seminal vesicles, forms the ejaculatory duct. A vasectomy involves the removal of a segment of the vas deferens to prevent the release of sperm during ejaculation, thereby preventing fertilization.