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The fusion of male gamete (sperm) and female gamete (egg) forms a zygote in sexual reproduction. The fusion of sperm with egg leads to the formation of the zygote is known as fertilization. Further, the zygotes develop to form an embryo. Fertilization can occur either outside the body or inside the female in animals. After fertilization, the egg can mature either inside or outside the female body. Generally, the egg has a protective covering over it if it develops outside the female body. All the animals present on the earth have different anatomical structures. Based on this the method of fertilization varied from species to species. Out of these, some lay eggs after fertilization, and others hold the egg inside the body till the completion of development. Birds lay an egg while mammals retain the egg inside the body till growth and development are completed.. In this tutorial, we will learn about fertilization in depth.
What is fertilization?
The fusion of sperm with egg leads to the formation of the zygote is known as fertilization. In higher animals or mammals, fertilization is considered to be a multi-step complex process. Generally, it is complete in 24 hours. In human, the male gamete which is known as sperm combine an egg or ovum from a female to form a zygote. At this point, pregnancy begins and takes a 280-day journey to complete the development of the fetus. Both the egg and sperm must undergo many steps to make the fertilization successful. The fertilized egg also undergoes several changes during this period.
How does fertilization start?
The male gamete sperms along with fluids making semen come out during ejaculation.
In every ejection, millions of sperms come out from the male reproductive organ but finally, only one sperm can be fertilized with the egg.
Eggs live in ovaries but only one egg is matured every month during ovulation every month.
It travels to the fallopian tube of the uterus.
The survival period of the egg is 24 hours.
If the egg is moving through the fallopian tube at the same time as sperms, then one sperm fuse with an egg. The survival of sperm in the uterus is 6 days. When the sperm cells join with an egg it is known as fertilization.
Stages of embryo development
The fusion of sperm with egg leads to the formation of the zygote via fertilization.
If the egg is moving through the fallopian tube at the same time as sperms, then one sperm fuse with an egg. The survival of sperm is 6 days to join with an egg.
The fertilized egg known zygote slowly moves towards the uterus from the fallopian tube.
During the growth and development phase, the zygote cell undergoes multiple divisions and forms a solid ball-like structure that is known as a morula.
The solid ball-like structure morula undergoes various changes to form a hollow ball known as a blastocyst. It gets into the uterus about 3-4 days after fertilization.
Shortly after fertilization, the embryo is produced from continuously dividing cells inside the blastocyst. Blastocyst constitutes the cells and fluids.
At the time of maturation a protective cover known as zona pellucida cover the blastocyst.
Beneath the zona pellucida, the outer cells are present that will further generate the placenta, and tissues that are present nearby support fetal development in the uterus.
There are changes happening in the inner layer of the blastocyst and its converted into tissues and finally developed into organs of the human body like bones, muscles, skin, liver, and heart.
The cells of the blastocyst undergo many changes and rapid growth has happened in these cells and converted them into more specialized cells.
Blastocyst implants in the endometrium at the movement when it outreaches the uterus.
The endometrium is an inner layer that lines the uterus.
The embryo is formed when the blastocyst reaches the finishing stage of the implementation process into the inner lining of the uterus.
The placenta is fully functional in this stage.
This is the time all the internal organs and external structures develop.
Cartilage is replaced by the first bone.
The embryo proceeds into an end stage of development by the twelve weeks of fertilisation. This stage is known as the fetal stage.
We can call the fetus a baby at around eight months.
The baby can move its whole body towards the end of nine months.
Development of the fetus
At a time when the embryo is 9 weeks old then it is called a fetus.
This time the fetus looks like a bean and the tail is no more present.
Reproductive organs start to differentiate.
The embryo progress to the ultimate stage of development by the twelve weeks of fertilisation. This stage is called the fetal stage. Though almost all the organs are created they need further development.
The urinary system and circulatory system of the fetus finish its growth by the end of twelve weeks.
The fetus can acknowledge stimuli and sound around twenty-four weeks.
We can call the fetus a baby at around eight months.
The baby can move its whole body towards the end of nine months and is ready to come out from the mother’s womb.
Stage by stage development
The germinal period
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The fusion of sperm with egg leads to the formation of the zygote is known as fertilization. The embryo is formed from the fertilized zygote. Fertilization can be happened either inside the female body or outside of it. The growth and development of the fetus take place in the mother's womb in viviparous animals. It takes a 280-day journey to complete the development of the fetus.
Q1. What is a germinal period?
Ans. The germinal period is the shortest and first stage of fetus development. The stage is last for approximately 9-12 days. It starts with a fusion of sperm and egg in the fallopian tube to form a zygote. The zygote undergoes various changes to form morula then its leads to blastocyst. The germinal period's end stage is the blastocyst implantation in the uterus.
Q2. What is the uterus?
Ans. It is a female reproductive organ. The growth and development fetus takes place here.
Q3. What is a fetal period?
Ans. The period of development of the fetus from 9 weeks to 40 weeks is known as a fetal period.
Q4. What is the function of the placenta?
Ans. Placenta supplies nutrients and oxygen from the mother’s womb to the fetus with help of the umbilical cord.
Q5. What is the duration of the embryonic period?
Ans. The embryonic period starts once the zygote is implanted in the uterine walls and lasts for eight weeks.
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