The structure indicated is the seminal vesicle.
The seminal vesicles are paired accessory glands of the male reproductive system. The seminal vesicles lie lateral to the ductus deferens just beneath the bladder. The excretory duct of the seminal vesicles join the ipsilateral vas deferens to form the ejaculatory ducts. The ejaculatory ducts formed on either side then pass through the prostate to open into the prostatic urethra at the colliculus seminalis.
The seminal vesicles contribute a significant volume of the fluid which forms semen.
Learn about the basic anatomy and structure of the male reproductive system in this anatomy tutorial.