An embryo is an organism in the early stages of development which cannot survive on its own. The precise definition of it varies; in humans, for example, a fertilized egg may be considered an embryo until around the eighth week of pregnancy, at which point it is termed a fetus. Embryos in animals typically indicate any pre-birth stage of development, including those in wombs or eggs. Plant embryos can take a number of different forms, though they are commonly encased in seeds.
The term “embryo” is only used to refer to "eukaryote" or multi-cellular organisms. Typically, people use the term specifically to refer to diploid eukaryotes, which have a complete set of genetic material from two donors. This genetic material takes the form of haploid sperm and eggs; a haploid cell only contains half a set of chromosomes, meaning that it cannot develop into anything unless it is combined with another one.
Formation of an embryo starts at fertilization. When an egg and sperm meet, they form a "zygote." A zygote is a single diploid cell, created through the merging of two haploid cells. After fertilization, the zygote starts to divide, laying the groundwork for the mature organism which will eventually be born, hatched, or grown. When this division begins, zygotes turns into embryos.
The status of embryos in humans is rather complex. At conception, a human egg and sperm become a zygote, which begins to divide, becoming an embryo. There is a great deal of debate regarding the definition of human embryos in terms of "life." This debate has led to conflict in many parts of the world, especially when it comes to terminating a pregnancy. As a human embryo matures, it starts to turn into a recognizable form, at which point people refer to it as a fetus.
Development in Animals
Embryos cannot survive independently because they lack the tissues, body structure, and organs needed to do so. The parent of an embryo must feed it and watch out for it until it reaches viability. In mammals, this is accomplished by incubating it inside the body and nourishing it with nutrients from the parents. Egg-laying animals provide the embryo with a rich layer of nutrients encased in a hard shell, which protects it until it is ready to be born.
Development of embryos in plants depends upon the way in which different species replicate. Those that utilize seeds include small structures that turn into the leaves, roots, and stem, which make up the embryos of these plants. Other species produce embryos as tiny plants that grow along the larger plant before separating off on their own to continue growing.