For example, the gland's tissue that makes the hormones is the parenchyma, whereas the stroma includes the nerves that innervate the parenchyma, the blood vessels that oxygenate and nourish it and carry away its metabolic wastes, and the connective tissues that provide a suitable place for it to be situated and anchored.

The main tissues that make up an organ tend to have common embryologic origins, such as arising from the same germ layer.

[10] The organ level of organisation in animals can be first detected in flatworms and the more derived phyla, i.e. the bilaterians.

The less-advanced taxa (i.e. Placozoa, Porifera, Ctenophora and Cnidaria) do not show unification of their tissues into organs.

[citation needed] In conifers, the organ that bears the reproductive structures is called a cone.

In other divisions (phyla) of plants, the reproductive organs are called strobili, in Lycopodiophyta, or simply gametophores in mosses.

Common organ system designations in plants include the differentiation of shoot and root.

[14] Both the source and method of obtaining the organ to transplant are major ethical issues to consider, and because organs as resources for transplant are always more limited than demand for them, various notions of justice, including distributive justice, are developed in the ethical analysis.

This situation continues as long as transplantation relies upon organ donors rather than technological innovation, testing, and industrial manufacturing.

[citation needed] The English word "organ" dates back to the twelfth century and refers to any musical instrument.

By the late 14th century, the musical term's meaning had narrowed to refer specifically to the keyboard-based instrument.

Historically, viscera of animals were examined by Roman pagan priests like the haruspices or the augurs in order to divine the future by their shape, dimensions or other factors.

[19] Aristotle used the word frequently in his philosophy, both to describe the organs of plants or animals (e.g. the roots of a tree, the heart or liver of an animal) because, in ancient Greek, the word 'organon' means 'tool', and Aristotle believed that the organs of the body were tools for us by means of which we can do things.

The yin and yang distinction approximates the modern notion of solid and hollow organs.