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Allantois: Function & Definition

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  • 0:00 What Are Amniotes?
  • 1:01 Allantois Defined
  • 2:03 Structure and Function
  • 3:25 Lesson Summary
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Lesson Transcript
Instructor: Jeremy Battista
The allantois can be found within the structure of an embryo, and in that way aids in the development of many organisms, including humans. In this lesson, learn about its function and definition.

What Are Amniotes?

The growth of an embryo is an amazing thing. From the most simplistic forms of life to the most complex, it is interesting how each grows from a bundle of cells into a living being. In humans especially, we see numerous stages and features that allow us to be the complex individuals that we are. One of these features is known as the allantois. Before we get into a definition of the allantois, however, it's important for you to understand which organisms use them.

Amniotes are a clade, or branch of animals which are similar to one another based on their eggs. Their basic distinguishing feature is that they lay their eggs on land or retain them inside of the mother. They contain an amnion, which is the protective covering on the embryo. This fills with amniotic fluid, allowing the embryo to grow and expand. This is a very important feature as terrestrial animals would have to deal with drying out, whereas those animals that are ocean-bound could lay their eggs without fear of them drying out.

Allantois Defined

The allantois is part of the developing embryo's structure. It helps in the exchange of gases and handles the liquid wastes from the developing organism. It is also one of the three structures that help to identify the amniotes from other living creatures. The other two are the chorion (membrane between mother and fetus) and the amnion (protective covering of the fetus).

The allantois is slightly different in each of the subgroups where it forms. An allantois for reptiles and birds differs slightly than an allantois for marsupials, which differs from the allantois of placental mammals. Remember, in reptiles and birds the embryo develops in an egg outside of the mother. In marsupials they develop in a pouch, and in placental mammals the embryo develops inside of the uterus of the mother.

While there may be differences in the development of the allantois, the main function remains almost the same. It is used to take up the nitrogenous (nitrogen containing) wastes (urine) of the embryo and aids in gas exchange.

Structure and Function

The allantois is a sac-like structure that forms near the posterior of the embryo. It eventually forms into the umbilical cord, but only in placental animals. The umbilical cord is how the offspring gets gases and nutrients. This is exactly what the allantois is used for. It is crisscrossed with blood vessels that help with the exchange. In egg-laying animals, the allantois facilitates oxygen exchange via the egg shell.

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