Reproduction is a complex phenomenon, when observed scientifically. The fertilization of the sperm and the oocyte create a single cell called zygote, which then divides and grows into a human baby. It is only the one single cell from which emerge all the complex systems that work to sustain the human body. The prenatal development in humans can be divided into two stages – embryonic development and fetal period. As soon as the fertilization occurs, the embryonic period begins. The human embryonic development has been divided into several stages...
...These stages of embryonic development are based on certain morphological features. These can be defined as a series of events that lead to the development of the human embryo.
This is the first step in the formation of the embryo. The female egg, when fused with the male sperm, becomes a zygote, a single cell structure that is formed by the fusion of the genetic material from both oocyte and the sperm. A total of twenty three chromosomes each are inherited from the sperm as well as the ovum. Together, these become a total of 46 chromosomes that begin to undergo changes before the mitosis can begin. It is the cell division, known as mitosis that creates an embryo consisting of two cells. The two cells, also known as the daughter cells, are surrounded by a glycoprotein called zona pellucida. The cells keep on dividing to form a spherical mass known as blastocyst. This blastocyst is made up of three different layers. Each of these layers forms different organs. The outermost layer, known as the ectoderm, forms the nervous system and the skin. The second layer, known as the endoderm, forms the digestive and the respiratory system and the innermost layer, known as mesoderm forms the muscle and skeletal system.
The mass of cells is now known as the embryo. This stage begins after the cell differentiation into the three layers of the blastocyst is complete. The umbilical cord and the placenta develop during this stage. Also, it is now that the cells begin to differentiate into various different organs and structures. By the time the embryonic stage ends, the fingers, mouth, eyes and ears begin to develop, marking the early embryonic development.
This stage begins when the cell differentiation is in the last stages. The embryo is now called a fetus. The neural tube begins to develop. This marks the start of the embryonic brain development. Other organs like the spinal chord and the rest of the nervous system develops fully. Embryonic heart development also begins during this stage. In the third month of the gestation, the sex organs begin to develop too. The fetus continues to grow in length and weight.