External fertilization occurs outside the body of the female. Internal fertilization occurs inside the body of the female. All terrestrial animals utilize internal fertilization, whereas most aquatic animals with the exception or marine mammals and some sharks utilize external fertilization. The presence of water is the major factor in whether internal or external fertilization is utilized. Conjugation is a process by which some lower animals like paramecium can exchange nuclear material between two individuals.
Development starts at the time of fertilization when a zygote forms in the fallopian tubes.
The zygote forms a protective membrane called a fertilization membrane which prevents any more sperm from penetrating the egg cell. At this point the zygote begins to divide by a process called cleavage. Cleavage ends up producing a blastula, a hollow ball of cells. This blastula then undergoes the process of gastrulation. At this point a process called differentiation occurs by which each cell layer develops into different body systems.