Zygote vs Fetus

The terms “zygote” and “fetus” are used to describe and label the stages of development of an organism. These two labels are often used in mammals which include human beings.
Both stages of development refer to the organism while it is inside a womb and before the actual delivery or birth.

“Zygote” is term used for the earliest and first stage of development following fertilization. Fertilization is the process where a sperm cell from a male contributor and an egg cell from a female contributor are united and combined into a single cell. The union of both egg cell and sperm cell (scientifically known as gametes) gives the zygote 26 chromosomes with each set coming from both parents. In this stage, the organism already has the DNA or genetic blueprint.

A zygote normally lasts for a week and then develops into a blastocyst and other stages of development. A few days after its conception, the zygote attaches to the mother’s womb while it grows and develops.

In terms of size, a zygote is only a single cell and can only be viewed by using a microscope. The zygote doesn’t increase its size and volume while it undergoes division. It only changes when the zygote turns into an embryo, the next stage of development.

In this stage, the organism is in its early stages of life, and there is no visible or concrete development except for the subsequent cell processes like division and reproduction of the original cell.

Another phenomenon that happens during this stage is the formation of twins or other multiples. As the zygote divides and forms cleavage, a single zygote can have the possibility of turning into twins or multiple children.

On the other hand, a fetus is the term for the last development stage of an organism. The term “fetus” is attached to an organism after a specific period of time (either when months or weeks) has passed. The fetus stage occurs after the embryo stage wherein the organism is almost fully formed and ready to leave the womb.

At the last stage of development, the fetus’ body formation and development are almost complete. Vital organs and bones are already formed, and there is a higher chance that the organism will survive the birth. Hair growth is visible on the head and eyelashes on the eyelids. The fetus now can have “movements” like kicking or flexing of its fingers or swallowing while still inside the womb.

Another feature of the fetus is that it reacts to environmental stimuli like light and sound.
The zygote and fetus are both important stages of development in any organism.

Summary:

1.“Zygote” and “fetus” are two labels for stages and development of an organism, especially mammals.
2.A zygote begins after fertilization which is the fusion of a sperm cell from the father and an egg cell from the mother. This stage occurs for a week or less after fertilization. On the other hand, the fetus begins at the seventh or eighth week of gestation; the fetus comes after the embryonic stage of development. It is also the last stage before the young is delivered.
3.In the zygote stage, the organism is still basically a cell that undergoes cleavage and division without changing its size and volume. In contrast, the fetal stage features a young with a distinct form and body.
4.A single zygote, in the process of cleavage, can form into identical twins or multiples. Meanwhile, a fetus only undergoes internal development and finishing its last moments of growth. It cannot develop further in terms of numbers.
5.A zygote can be seen by a microscope and isn’t visible in the mother’s body. A fetus, however, can be seen by an ultrasound machine and is visible by a growing protrusion in the mother.

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