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Embryonic Stages of Development

Embryonic Stages of Development

Embryonic development, also known as embryogenesis, is the process by which an embryo develops or forms. Cell division and cellular differentiation occur during the early stages of embryonic development in humans. In humans, embryonic development goes on for the first eight weeks because the embryo is termed a fetus by the ninth week. As the embryo continues to develop, the germ layer is formed as cells differentiate through a process known as gastrulation. Typically, most animals are either diploblastic or triploblastic. Diploblastic animals, such as jellyfish, form two embryonic germ layers, while triploblastic animals, such as humans, form three embryonic germ layers.

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The Ectoderm, Mesoderm, And Endoderm

Consistently, the three germ layers in humans include the ectoderm, mesoderm, and endoderm (MacCord, 2013b). The ectoderm is the outermost layer of the germ cells and is divided into the neural and the external ectoderm. The ectoderm differentiates to form neural tissues such as the brain, the spine, and peripheral nerves. It also forms epithelial tissues such as the skin, hair, nails, mouth lining, and the anus. The mesoderm is the mid-layer of the germ tissues. It gives rise to the mesenchyme, non-epithelial blood cells, skeletal muscles, heart, and mesothelium (MacCord, 2013c). Lastly, the endoderm is the innermost layer of the germ layers. At first, the endoderm consists of cells that are flattened, which later become columnar. Its function is to form linings for the digestive and respiratory tubes (MacCord, 2013a).

Structure And Function Of The Embryonic Disk

An embryonic disc, also known as a blastodisc, is part of the inner cell mass of a blastocyst that develops during the second prenatal week. It is sandwiched between the amnion and the yolk sac. The embryo of any mammal with a placenta develops from an embryonic disk. After the embryonic disk has developed, it acquires the three germ layers: the ectoderm, mesoderm, and endoderm.

Embryo Development

Subsequently, prenatal development has three major stages. The stages include the germinal period, the embryonic period, and the fetal period. The germinal period is the first stage in prenatal development and lasts for about eight to nine days. It begins at conception when the egg is fertilized to form a zygote and ends when the blastocyst attaches to the inner wall of the uterus, known as the endometrium. The embryonic stage begins the third week after conception and results in embryo formation. Unlike in the gestational period, where there is just a mass of cells, the embryo is more defined as a human in the embryonic stage. This stage is crucial for the development of the brain. Lastly, the fetal period is the longest in prenatal development. It begins from the 9th week after fertilization and ends at birth.

The Role Of Chorionic Villi

Villi are tiny vascular projections that provide a larger surface area of a membrane. Chorionic villi are villi found in the placenta and play an essential role in providing a larger surface area for which maternal blood products are made available to the fetus. Since the chorionic villi have the same genetic matter as the fetus, they can be collected to examine whether a fetus has any genetic disorder.

Environmental Factors That May Cause Malformations In Embryo Development

During development, the embryo is predisposed to malformation triggered by environmental factors. These environmental factors include tobacco smoke, air pollutants, chemicals, and pesticides (Weinhold, 2022). When mothers are exposed to these factors, it results in low birthweight, congenital abnormalities, stillbirth, and even preterm births. These environmental factors also put the mother at risk of high blood pressure, fibroids, and increased chances of breast cancer.


MacCord, K. (2013a). Endoderm | The Embryo Project Encyclopedia. Retrieved 17 August 2022, from

MacCord, K. (2013b). Germ Layers | The Embryo Project Encyclopedia. Retrieved 17 August 2022, from

MacCord, K. (2013c). Mesoderm | The Embryo Project Encyclopedia. Retrieved 17 August 2022, from

Weinhold, B., 2022. Environmental Factors in Birth Defects: What We Need to Know. PubMed Central (PMC). Retrieved August 17 from


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In a one to two-page paper, give an overview of the embryonic stage of development.

Embryonic Stages of Development

Embryonic Stages of Development

Describe the ectoderm, mesoderm, and endoderm and their associated structures.
Discuss the structure and function of the embryonic disk.
Describe embryo development and the role of chorionic villi.
Include any environmental factors that may cause malformations in the developing embryo.

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