seminal vesicle, either of the paired saclike glandular structures posterolateral to the urinary bladder in the male and functioning as part of the reproductive system. Each sac is pyramidal in shape and convoluted in appearance and at the anterior extremity becomes constricted into a narrow straight duct that joins the vas deferens to form the ejaculatory duct. The seminal vesicles produce a fluid that is added to the secretion of the testes and other glands to form the semen.
