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(from Greek ἔμβρυον, embryon, "the unborn, embryo"; and -λογία, -logia) is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage.
Embryonic development of animals

After cleavage, the dividing cells, or morula, becomes a hollow ball, or blastula, which develops a hole or pore at one end.


In bilateral animals, the blastula develops in one of two ways that divides the whole animal kingdom into two halves (see: Embryological origins of the mouth and anus).
If in the blastula the first pore (blastopore) becomes the mouth of the animal, it is a protostome; if the first pore becomes the anus then it is a deuterostome. The protostomes include most invertebrate animals, such as insects, worms and molluscs, while the deuterostomes include the vertebrates. In due course, the blastula changes into a more differentiated structure called the gastrula.
The gastrula with its blastopore soon develops three distinct layers of cells (the germ layers) from which all the bodily organs and tissues then develop:
The innermost layer, or endoderm, gives rise to the digestive organs, the gills, lungs or swim bladder if present, and kidneys or nephrites.
The middle layer, or mesoderm, gives rise to the muscles, skeleton if any, and blood system.
The outer layer of cells, or ectoderm, gives rise to the nervous system, including the brain, and skin or carapace and hair, bristles, or scales.
Embryos in many species often appear similar to one another in early developmental stages. The reason for this similarity is because species have a shared evolutionary history. These similarities among species are called homologous structures, which are structures that have the same or similar function and mechanism, having evolved from a common ancestor.


Humans are bilaterans and deuterostomes—the anus develops first.
In humans, the term embryo refers to the ball of dividing cells from the moment the zygote implants itself in the uterus wall until the end of the eighth week after conception. Beyond the eighth week after conception (tenth week of pregnancy), the developing human is then called a fetus.

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