Male Reproductive System

Most species have two sexes: male and female. Each sex has its own unique reproductive system. They are different in shape and structure. Male has reproductive organs, or genitals, that are both inside and outside the pelvis. The male genitals include: testicles, duct system, accessory glands and penis
In a guy who's reached sexual maturity, the two testicles produce and store millions of tiny sperm cells. The testicles are also part of the endocrine system because they produce hormones, including testosterone which is a major part of puberty in guys. Alongside the testicles are the epididymis and the vas deferens, which make up the duct system of the male reproductive organs. The accessory glands, including the seminal vesicles and the prostate gland, provide fluids that lubricate the duct system and nourish the sperm. The seminal vesicles are sac-like structures attached to the vas deferens to the side of the bladder. The prostate gland, which produces some of the parts of semen, surrounds the ejaculatory ducts at the base of the urethra.
The sperm then move to the vas deferens, or sperm duct. The seminal vesicles and prostate gland produce a whitish fluid called seminal fluid, which mixes with sperm to form semen when a male is sexually stimulated.

Spermatogenesis

Spermatogenesis is the process by which male primary germ cells undergo division, and produce a number of cells termed spermatogonia, from which the primary spermatocytes are derived. Each primary spermatocyte divides into two secondary spermatocytes, and each secondary spermatocyte into two spermatids or young spermatozoa. These develop into mature spermatozoa, also known as sperm cells. Spermatozoa are the mature male gametes in many sexually reproducing organisms. Thus, spermatogenesis is the male version of gametogenesis.

In mammals it occurs in the male testes and epididymis in a stepwise fashion, and for humans takes approximately 74 days. Spermatogenesis is highly dependent upon optimal conditions for the process to occur correctly, and is essential for sexual reproduction. It starts at puberty and usually continues uninterrupted until death, although a slight decrease can be discerned in the quantity of produced sperm with increase in age. The entire process can be broken up into several distinct stages, each corresponding to a particular type of cell.

Spermiogenesis

During spermiogenesis, the spermatids begin to grow a tail, and develop a thickened mid-piece. Spermatid DNA also undergoes packaging, becoming highly condensed. The Golgi apparatus surrounds the now condensed nucleus, becoming the acrosome. One of the centrioles of the cell elongates to become the tail of the sperm. The resulting spermatozoa are now mature but lack motility, rendering them sterile. The mature spermatozoa are released from
the protective sertoli cells into the lumen of the seminiferous tubule in a process called spermiation.
The non-motile spermatozoa are transported to the epididymis in testicular fluid secreted by the sertoli cells with the aid of peristaltic contraction.

Ejaculation

Ejaculation begins during the first or second contraction of orgasm. The first or second spurt is typically the largest and can contain 40 percent or more of the total ejaculates volume. After this peak the flow of each pulse diminishes. When the flow ends, the muscle contractions of the orgasm continue with no additional semen discharge.

Semen and Composition of human semen

Semen is an organic fluid, also known as seminal fluid, that may contains spermatozoa.
It is secreted by the gonads (sexual glands) and other sexual organs of male which can fertilize female ova. During the process of ejaculation, sperm passes through the ejaculatory ducts and mixes with fluids from the seminal vesicles, the prostate, and the bulbourethral glands to form the semen. The seminal vesicles produce a yellowish viscous fluid rich in fructose and other substances that makes up about 70% of human semen. The prostatic secretion, influenced by dihydrotestosterone, is a whitish (sometimes clear), thin fluid containing proteolytic enzymes, citric acid, acid phosphatase and lipids. The bulbourethral glands secrete a clear secretion into the lumen of the urethra to lubricate it.

Appearance and consistency of human semen

Semen is typically translucent with white, grey or even yellowish tint. After ejaculation, the latter part of the ejaculated semen coagulates immediately, forming globules, while the earlier part of the ejaculate typically does not. After a period typically ranging from 15 – 30 minutes, Prostate-specific antigen present in the semen causes the decoagulation of the seminal coagulum. It is postulated that the initial clotting helps keep the semen in the vagina, while liquefaction frees the sperm to make their journey to the ova.

Reference

http://kidshealth.org/teen/sexual_health/changing_body/male_repro.html
http://en.wikipedia.org/wiki/Spermatogenesis
http://en.wikipedia.org/wiki/Semen