Sexual Reproduction Process
The process of sexual reproduction involves two parents.   Both parents normally contribute one gamete or sex cell to the process.   This process assures that the genetic information given to the offspring will be obtained equally from each parent.  The female gamete is called the egg or the ovum and the male gamete is called a sperm. These gametes are formed in specialized reproductive structures called gonads.   The sperm is much smaller than the egg, but is capable of moving on its own power using a whip-like tail called a flagellum.

Sperm and Egg (fertilization)

The sperm and egg unite in a process called fertilization.   This process forms a single celled structure called a zygote which contains the complete genetic information to develop into a complete new organism having characteristics of both parents.

Process of Fertilization

This zygote will then divide by mitosis and form the specialized cells, tissues, and organs of the organism.   This development of specialized structures from the zygote is called differentiation.

Meiosis
The process of meiosis produces gametes or sex cells.   While some parts of this cell division process are similar to the asexual cell division process of mitosis, there are several key differences.    Meiosis produces gametes, while mitosis produces other cell types.   The process of meiosis halves the chromosome number from the original parent cell in the four cells it forms.   It does this by having two cell divisions forming four cells, where mitosis has only one cell division forming two cells.    Both processes start out with one doubling or replication of the chromosome material.   The graphic below will help to visually illustrate some of the key events of meiosis.

Process of Meiosis

Another important way that meiosis differs from mitosis is the exchange of chromosome pieces which occurs in the first division of this process.   This exchange of chromosome pieces is called crossing over.   Crossing over assures that the cells produced as a result of meiosis will be different from and exhibit variations from the parent cell that produced them.    This process is chiefly responsible for the variations seen in members of the same species of sexually reproducing organisms.    These variations are the driving force for the process of natural selection.

The process of crossing over and how it produces variation when these chromosomes are 
recombined in the process of fertilization is illustrated in the graphic below.

Crossing Over and Genetic Recombination

Comparative Reproduction and Development
Different organisms possess different adaptations for reproduction and development.   Organisms which spend their lives or a large proportion of their lives in the water tend to lay their eggs in great numbers (thousands) in the water and wait for the male of the species to release sperm near them to fertilize them.   The fertilization which occurs in the water in this case outside the body of the organism is called external fertilization.  These young organisms then develop outside the mother in the water once this has occurred, which is called external development.  A disadvantage of this process is that the eggs and developing young have little or no parental protection.    Many fish and amphibians like frogs undergo fertilization and development in this manner.

Reptiles and birds engage use the process of internal fertilization to fertilize their eggs.  In this situation, the male of the species inserts his sperm inside the female, who then lays her fertilized eggs outside her body.    The process of development is then external.  Reptiles and especially birds tend to lay fewer eggs and provide much more parental protection for their developing young.   Organisms (with some exceptions) which use the process of internal fertilization tend to spend much of their lives on land.  Mammals like humans have both their fertilization and initial stages of development occur within the female organism.   This is referred to as internal fertilization and internal development.   These organisms tend to release very few eggs, but those eggs and the developing organism are very well protected by one or both parents.