Embryonic Stage Development: Definition & Concept

Embryonic Stage Development: Definition & Concept
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  • 0:01 What Is Embryonic Development?
  • 1:07 Implantation
  • 2:35 Differentiation of Germ Layers
  • 3:35 Organogenesis
  • 4:17 The Embryo Becomes a Fetus
  • 5:11 Lesson Summary
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Lesson Transcript
Instructor: Hilary North

Hilary is a biomedical researcher with a PhD in neuroscience.

There are three basic stages of mammalian gestation: germinal, embryonic, and fetal. This lesson focuses on embryonic development and the stages of growth between implantation and the point when an embryo becomes a fetus.

What Is Embryonic Development?

During mammalian development, the time during which a young animal grows inside its mother is known as gestation. We all know that a human baby grows inside of the mother's uterus for a period of time - about 9 months - and that it has years of development ahead of it following birth. But, what actually occurs during gestation?

There are three general phases of gestation. The germinal stage is the period of gestation from fertilization or conception, when the egg meets the sperm, to implantation of the embryo in the uterus. The embryonic stage of gestation is the period after implantation, during which all of the major organs and structures within the growing mammal are formed. Once the embryo is fully formed, it expands, grows, and continues to develop in what is known as the fetal development stage. This is when the mother becomes physically enlarged and visibly pregnant! The fetal development stage concludes at birth.

Now, let's go over the major events during the embryonic stage of gestation.


Conception and fertilization are terms for the moment when the sperm fertilizes the egg. This occurs at some point following the egg's release from an ovary, on its journey to the uterus. Even from the moment of joining, changes are happening and the cells begin to split time and time again, as the now multi-cell clump continues on its journey to the womb.

All journeys must come to an end, and the little cell clump's story is no different. In order for development to continue, a connection must be formed between the mother and the embryo. Upon arrival to the uterus, the rapidly-changing embryo implants, or burrows, into the uterine lining. In human development, implantation happens 5-10 days after conception in almost all cases.

After fertilization but before implantation, an embryo is surrounded by its trophectoderm. The trophectoderm, or trophoblast, is a special layer of cells that covers the outside of an embryo and supplies the ball of cells with nutrition. Once implantation has occurred, this trophectoderm changes in significant ways. The trophectoderm begins to differentiate into supporting structures such as the placenta so that the embryo can now receive nutrition and oxygen from the mother's blood supply.

It is estimated that only half of fertilized eggs successfully implant in the uterine wall; when implantation fails, the pregnancy ends without the mother even knowing it had occurred in the first place.

Differentiation of Germ Layers

Around the time of implantation, the embryo becomes divided into germ layers. A germ layer is a collection of cells that determine which cells within the embryo will eventually develop into particular organs and tissues.

There are three germ layers: ectoderm, mesoderm, and endoderm. The ectoderm, towards the outer surface of the embryo, will eventually become the nervous system, skin, and sensory organs such as the ears and eye. It is easy to remember that these organs arise from ectoderm because ecto means 'outer' and organs such as the skin are towards the outside of the body.

Similarly, the mesoderm, just inside the outer-most ectoderm of the embryo, becomes the skeleton, muscles, circulatory system, and the genitalia. The endoderm, at the core of the embryo, becomes the most inner organs such as the gastrointestinal system and the lungs as well as the pancreas and liver.


In the final phases of embryonic development, the three germ layers begin to become partitioned into distinct segments that will become various organs and structures. This process is known as organogenesis, or, literally, the generation of organs.

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