When gametes join they form a cell called a zygote. Human sperm and eggs contain 23 chromosomes. Human zygotes contain 46 chromosomes. The type of cell division that produces gametes with half the normal chromosomes is called meiosis.  

During oogenesis—at the end of meiosis and cytokinesis—an oogonium divides into one mature ovum capable of being fertilized and three polar bodies that are reabsorbed, while a spermatogonium divides into four viable spermatozoa capable of fertilization.

A spermatid is the final product of spermatogenesis. It is a haploid cell, meaning it has only one copy of each allele (one of each chromosome instead of two). Normal diploid cells have two copies of each chromosome, for a total of 46. Spermatids have half this number, for a total of 23 chromosomes. Each chromosome is composed of only a single chromatid following division, for a total of 23 chromatids.

When the zygote is formed during the fusion of the sperm and egg cells, the final cell is diploid, containing 46 chromosomes (23 from each gamete).

An outline of spermatogenesis is given here for further understanding.

Spermatogenesis Timeline:

1) Spermatogonium (46 chromosomes, 92 chromatids)

- Has a pair of each chromosome, and each individual chromosome has two chromatids.

- Undergoes mitosis (normal cell division) to produce a primary speratocyte.

2) Primary spermatocyte (46 chromosomes, 46 chromatids)

- Has a pair of each chromosome and each individual chromosome has one chromatid.

- Then replicates its DNA, resulting in 46 chromosomes with 92 chromatids.

- Then undergoes meiosis I (homologous chromosome pairs separate), producing two secondary spermatocytes.

3) Secondary spermatocytes (23 chromosomes, 46 chromatids)

- Has one of each chromosome, and each individual chromosome has two chromatids.

- Each secondart spermatocyte undergoes meiosis II (chromatids of each chromosome separate, similar to mitosis), producing a total of four spermatids.

4) Spermatids (23 chromosomes, 23 chromatids).

- Has one of each chromosome, and each individual chromosome has one chromatid

Spermatogenesis occurs only in human males (in the testes), not in females. It is the process by which spermatids are formed via meiosis. The process of meiosis contributes to genetic diversity. This is why siblings are not identical to each other or to their parents.

Spermatogenesis is the process of producing the male gametes, spermatozoa (sperm). The spermatogonia in the seminiferous tubules of the testes mature into spermatocytes, which undergo meiosis to form spermatozoa. The spermatozoa complete their maturation process in the epididymas, then they are ready to enter the vas deferens before ejaculation. Lumen is a general term that means "an opening." Although spermatogenesis does occur in the lumen of the seminiferous tubules, the term lumen is too broad and is not the best answer. An ampulla is a general term that, in anatomy, means "the dilated end of a duct." There is an ampulla of the vas deferens before it enters the prostate gland, but spermatogenesis does not occur there. The prostate gland produces a milky, alkaline solution that comprises about 30% of semen. The seminal vesicles are glands that produce the majority of the solution that will become semen. Note the difference between semen (fluid) and sperm (cells).

Follicle-stimulating hormone may help to sequester testosterone in the testes (the Sertoli cells can perform the same function by releasing androgen-binding protein), but the only hormone needed to maintain spermatogenesis is testosterone itself. FSH is not needed for in-vitro maturation of spermatozoa, and thus cannot be said to be required inside the body for the process of spermatogenesis.

A very high level of testosterone (15-70 times greater than in the blood) is required for the initiation and maintenance of spermatogenesis, and androgen-binding protein's most direct role is ensuring this high concentration of testosterone by preventing absorption of testosterone back into the body across the blood-testis barrier.

The S phase (or synthesis phase) is a period of the cell cycle during which DNA is replicated (or synthesized). G1 and G2 are both growth phases, during which cellular organelles are replicated and the cell grows in size. The M phase refers to mitosis, or cell division.

During the S phase, the cell dedicates its time to replicating its DNA. The S phase is named for the synthesis of DNA.

The G phases are needed for increasing proteins and cellular organelles, and are known for cellular growth. The M phase is also known as mitosis.

There are two stages in the cell cycle marked by the replication of organelles and protein synthesis: G1 and G2. G1 follows mitosis and allows the cell to grow. G2 occurs just before mitosis, and ensures that both daughter cells will have adequate organelles. It also allows proteins necessary for mitosis to be translated.

The overall order of the cell cycle is: G1, S, G2, M. G1 is a growth period. S marks the replication of DNA, resulting in the production of identical sister chromatids. G2 is responsible for organelle synthesis. The M phase is mitosis, or cell division.