A gastrula is a developmental stage of some animal embryos in which different germ layers are present. In this lesson, learn about the structure of a gastrula, how it forms, and what its germ layers become in adult humans.


During early animal development, most embryos develop into a blastula or hollow ball of cells. Then the embryos form a gastrula by a process called gastrulation. During gastrulation, some of the outer embryonic cells get pushed inward during a process called invagination. This creates many structures, including germ layers. We’ll come back to what germ layers are a little bit later. Right now, let’s look at the process of gastrulation and the structures it forms.


You can imagine what happens during gastrulation by thinking about a balloon. A normal blown up balloon is like a blastula. The outer rubbery surface of your balloon is equivalent to the outer layer of cells in a blastula. The part of your balloon filled with air is like the inner hollow part of a blastula called a blastocoel. If you take your finger and push in on the balloon as far as it goes, you’ll have a newly formed pocket around your finger. Also, you just demonstrated invagination.

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Parts of a Gastrula

Your balloon is now like a gastrula or an embryo that has undergone gastrulation and has germ layers. Let’s take a look at the different parts of a gastrula. Invagination during gastrulation produces a pocket or space called the archenteron. This is just like the pocket you formed in your balloon, and it will develop into the gut or digestive system of an animal.

The opening of the archenteron is called the blastopore. Imagine if you removed your finger from the balloon, but the balloon kept its shape. There would be a hole or opening there, just like the blastopore. Depending on the type of animal, a blastopore will become either the mouth or the anus. The part of the balloon surrounding your newly formed pocket and that was touching your finger is a new inner layer or germ layer. So what exactly is a germ layer?

Germ Layers

Germ layers, produced during gastrulation, are the initial tissue layers of an embryo. Sometimes they’re called embryonic tissue layers. They will develop into all the different tissues and structures present in an adult animal. Animals can have a maximum of three germ layers:

  1. Ectoderm: outermost layer of cells
  2. Mesoderm: middle layer of tissues
  3. Endoderm: innermost layer of cells

Animals like cnidarians (including corals, jellyfish, and sea anemones) and ctenophores (such as comb jellies) have only the ectoderm and endoderm layers, which makes them diploblastic. Other animals, like humans, are triploblastic, or have all three germ layers. Only in triploblastic animals have a mesoderm.

In our balloon example, the outer surface of the balloon represents the ectoderm. The new inner surface you created by invagination is like the endoderm. However, the process of gastrulation in triploblastic animals is a little more complex than this because mesoderm must also form. In addition to some of the outer embryonic cells undergoing invagination, some of these cells also break off inside the blastocoel to form the mesoderm.

Derivatives in Humans

Any tissue in an adult animal must be derived from one of the three embryonic germ layers. Using humans as an example, let’s look at what structures each of the three germ layers will develop into.

The ectoderm is the outer layer of cells in a gastrula. In humans, it will develop into mostly external structures and a few internal structures, including:

  • Hair
  • Nails
  • Skin
  • Tooth enamel
  • Brain
  • Spinal cord

The endoderm is the inner layer of cells in a gastrula, so it develops into the innermost tissues of the body. The following are some derivatives of the endoderm in humans:

  • Inner lining of digestive and respiratory tracts
  • Liver
  • Pancreas
  • Part of the urinary tract

The mesoderm is the middle layer of cells in a gastrula. It develops into most internal structures in the human body, including some organs from each of the following body systems:

  • Cardiovascular
  • Endocrine
  • Lymphatic
  • Muscular
  • Reproductive
  • Skeletal
  • Urinary

Lesson Summary

Animals that undergo gastrulation during development will form a gastrula with two (diploblastic) or three (triploblastic) different germ layers. Additionally, a gastrula has an opening called a blastopore and a cavity called the archenteron. The archenteron will become the gut of the animal. The embryonic tissue layers of the gastrula will eventually develop into the tissues and structures found in an adult animal. The outer ectoderm will become your skin and nervous system. The inner endoderm will form linings of the digestive and respiratory tracts. The middle mesoderm layer will form many structures in a majority of your organ systems.