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. The endoderm develops into the inner lining of the digestive tract, glands including liver and pancreas, inner lining of the respiratory system. At this point further development occurs until the organism develops to the point of birth or hatching. Development then continues until the organism matures.