The proximal segment of the urethra is surrounded by the _prostate gland_. The secretion of this gland is conveyed into the urethra by numerous short ducts, known as the _prostatic ducts_. Behind the prostate, at the base or fundus of bladder, are the paired _seminal vesicles_. The duct of the seminal vesicle joins the _vas deferens_ of the same side (both functionally and embryologically the seminal vesicle is no more than a diverticulum of the vas deferens); passing on under the name of the _common seminal_ or _ejaculatory duct_, the canal opens into the prostatic portion of the urethra (the orifices of the two common seminal ducts are in the folds of mucous membrane forming the right and left lateral margins of the _prostatic utricle_ or _uterus masculinus_). These ducts convey the secretion of the testicles into the urethra along which canal it passes to the exterior. Behind the posterior part of the urethra, but distal to the prostate gland, are situate also the paired _glands of Cowper_, or _suburethral glands_, whose excretory ducts likewise open into the urethra. There are glands also in the walls of the seminal vesicles, the vasa deferentia, and the urethra; the urethral glands are commonly known as the _glands of Littre_. As previously mentioned, it is in the testicles that the secretion necessary for the reproductive act is prepared. This secretion is evacuated during sexual intercourse, and also during masturbation and involuntary seminal emissions. The testicular secretion is a tenacious fluid. When examined microscopically, it is seen to contain countless spermatozoa, structures about 50 [Greek: m] (1/500 inch) in length, with a thick head and a long filiform tail. They represent the male reproductive cells, which during coitus are introduced into the interior of the female reproductive organs; a single spermatozoon unites with the ovum of the female to form the fertilised ovum. The spermatozoa are formed in the walls of the convoluted seminiferous tubules. The cells lining these tubules are of several different kinds (although in childhood they are not differentiated as they are after the puberal development has taken place). One variety of these cells, the _spermatogonia_, undergo an increase of size at puberty, and from these spermatogonia, after passing through several intermediate transitional stages, the spermatozoa are formed.