During sexual reproduction in animals, a haploid sperm and unites with a haploid egg cell to form a diploid zygote. The zygote divides mitotically and differentiates into an embryo. The embryo grows and matures. After birth or hatching, the animal develops into a mature adult capable of reproduction. Some invertebrates reproduce by self-fertilization, in which an animal's sperm fertilizes its own eggs. Self-fertilization is common in tapeworms and other internal parasites, which lack the opportunity to find a mate. Most animals, however, use cross fertilization, in which different individuals donate the egg and the sperm. Even hermaphrodites animals (such as the earthworms) that produce both types of gametes use cross-fertilization.
Animals exhibit two patterns for bringing sperm and eggs together. One is external fertilization, whereby animals shed eggs and sperm into the surrounding water. The flagellated sperm need an aquatic environment to swim to the eggs, the eggs require water to prevent drying out. Most aquatic invertebrates, most fish, and some amphibians use external fertilization. These animals release large numbers of sperm and eggs, thereby overcoming large losses of gametes in the water. In addition, courting behavior in some species brings about the simultaneous release of the gametes, which helps insure that sperm and egg meet.
The other pattern of sexual reproduction is internal fertilization, whereby the male introduces sperm inside the females reproductive tract where the eggs are fertilized. Internal fertilization is an adaption for life on land, for it reduces the loss of gametes that occurs during external fertilization. Sperms are provided with a fluid (semen) that provides an aquatic medium for the sperm to swim when inside the male's body. Mating behavior and reproductive readiness are coordinated and controlled by hormones so that sperm and egg are brought together at the appropriate time.
After internal fertilization, most reptiles and all birds lay eggs that are surrounded by a tough membrane or a shell. Their eggs have four membranes, the amnion, the allantois, the yolk sac and the chorion. The amnion contains the fluid surrounding the embryo; the allantois stores the embryo's urinary wastes and contains blood vessels that bring the embryo oxygen and take away carbon dioxide. The yolk sac holds stored food, and the chorion surrounds the embryo and the other membranes. After the mother lays her eggs, the young hatch.
Mammals employ internal fertilization, but except for the Australian montremes such as the duckbill platypus and the echidna, mammals do not lay eggs. The fertilized eggs of mammals implant in the uterus which develops into the placenta, where the growth and differentiation of the embryo occur. Embryonic nutrition and respiration occur by diffusion from the maternal bloodstream through the placenta. When development is complete, the birth process takes place.
See also Chromosome.
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