Zygote-Formation Of a New Being (Stages of Gestation) | ScienceMonk

“The union of sperm and ovum fuse to produce a cell during the process of conception. And one-celled organism resulting from fertilization is known as a zygote.”

Most adults and even most children in developed countries have a reasonably accurate idea of where babies come from. Only a generation or two, ago many parents told their children that a stroke had bought them.


Historical Perception of Fertilization-Zygote Formation

The folk belief that children came from wells, springs, or rocks was common everywhere as late as the beginning of the twentieth century. Conception was believed to be influenced by cosmic forces. A baby conceived under a new moon would be a boy; during the moon’s quarter, a girl.

During the seventeenth and eighteenth centuries, a debate raged between two schools of biological thought. Some people claimed that fully formed “babies” were contained in the heads of sperm, ready to grow when deposited in the nurturing environment of the womb.

The male sperms were then called as animalcules. So these people were called as animalculists. On the other hand, the ovists, inspired by influential work of English physician William Harvey, held on opposite but equally incorrect view: that a female’s ovaries contained tiny, already formed human whose growth was activated by the male’s sperm.

Finally, in the late eighteenth century; the German-born anatomist Kaspar Fredrich Wolff demonstrated that embryos are not pre-formed in either parent and that both contribute equally to the formation of a new being. “The union of sperm and ovum fuse to produce a cell during the process of conception. And one-celled organism resulting from fertilization is known as a zygote.”


Fertilization- Conceiving a New Life (Zygote)

Every cell in the healthy human body has 46 chromosomes except for gametes. Thus, when sperm and ovum fuse at conception, they produce a zygote with 46 chromosomes, 23 from mother and 23 from the father. Fertilization, or conception, is the process by which sperm and ovum combine to create a single cell called a zygote, which then duplicates itself again and again by cell division to become a baby.

What turns a fertilized ovum, or zygote, into a creature with specific shape and pattern? Research suggests that an identifiable group of genes is responsible for this transformation in vertebrates, presumably including human beings. These genes produce molecules called morphogens, which are switched on after fertilization and begin sculpting arms, hands, fingers, vertebrae, ribs, a brain, and other body parts.

Scientists are also learning about the environment inside the womb and how it affects the developing person. It is most likely if intercourse occurs on the day of ovulation or during the five days before. If fertilization does not occur, the ovum and any sperm cells in the women’s body die.


Pre-Natal Development:-From Zygote to Newborn Baby

After the formation of a zygote occurs, the development process gets started until the baby gets born. This period is called as gestation or prenatal development. It takes place in 3 stages: germinal (fertilization-2 months), embryonic (2-8 weeks), and fetal (8 weeks- birth).

During these three stages of gestation, the original single-celled zygote grows into an embryo and then a fetus. Both before and after birth, development proceeds according to two fundamental principles. The cephalocaudal principle dictates that development proceeds from head to the lower part of the trunk.

An embryo’s head, brain, and eyes develop earliest and disproportionately large until the other parts catch up. At two months of gestation, the embryo’s head is half the length of the body. According to the proximodistal principle, the development proceeds from parts near the center of the body to outer ones.

The embryo’s head and trunk develop before the limbs and the arms and legs before the finger and toes. The development process usually goes through Fertilization – cleavage- blastulation – implantation. The development after implantation as follows. 

3-4 Weeks-The Embryo reaches the size 10,000 times greater than the Zygote”

During the first month, growth is more rapid than at any other time during prenatal or postnatal life; the embryo reaches the size 10,000 times greater than the zygote. By the end of the first month, it measures about a ½ inch in length. Blood flows through its veins and arteries, which are very small. It has a minuscule heart, beating 65 times a minute. It already has the beginning of a brain, kidney, liver, and digestive tract. The umbilical cord, its lifeline to the mother, is working.

Seven Weeks

By the end of the second month, the organ is less than 1 inch long and weighs only 1/3 ounce. Its head is half its total body length. Facial parts are developed, with tongue and teeth buds. The arms have hands, fingers, and thumbs and the legs have knees, ankles, and toes. It has a thin covering of akin and can make handprints and footprints. Bone cells appear at about eight weeks. The skin is sensitive enough to react to tactile stimulation.

Three Months

By the end of the third month, the fetus weighs about 1 ounce and measures about 3 inches in length. It has fingernails, toenails, eyelids (still closed), vocal cords, lips, and a prominent nose. Its head is still large- about 1/3 its total length- and its forehead is high. Sex can be determined easily. The organ systems are functioning, and so the fetus may now breathe, swallow amniotic fluid into the lungs and expel it, and occasionally urinate.

Four Months

The body is catching up to the head, which is now only ¼ the total body length, the same proportion it will be at birth. The fetus now measures 8-10 inches and weighs about 6mounces. The umbilical cord is as long as the fetus and will continue to grow with it. The placenta is now fully developed. The mother may be able to feel the fetus kicking, a movement known as quickening.

Five Months

The fetus, now weighing about 12 ounces to 1 pound and measuring about 1 foot, begins to show signs of an individual personality. It has to define sleep-wake patterns, has a favorite position in the uterus, and becomes more active- kicking, stretching, squirming, and even hiccupping. By putting an ear to the mother’s abdomen, it is possible to hear the fetal heartbeat. Coarse hair has begun to grow for eyebrows and eyelashes, fine hair is on the head, and a woolly hair called lanugo covers the body.

Six months after the formation of Zygote

The rate of fetal growth has slowed down a little- by the end of the six months, the fetus is about 14 inches long and weighs 1 ¼ pound. It has fat pads under the skin; the eyes are a strong grip. A fetus born during the six months still has only a slight chance of survival, because the breathing apparatus has not matured. However, some fetuses of this age do survive outside the womb.

Seven Months

By the end of the seven months, the fetus, about 16 inches long and weighing 3-5 pounds, now has fully developed reflex patterns. It cries, breathes, swallows, and may suck its thumb. The language may disappear at about this time or may remain until shortly after birth. Head hair may continue to grow. The chance that a fetus weighing at least 3 ½ pounds will survive is relatively good, provided it receives intensive medical attention. It will probably need to be kept in an isolette until a weight of 5 pounds is attained.

Eight Months

The 8-month-old fetus is 18-20 inches long and weighs between 5-6 pounds. Its living quarters are becoming cramped, and so its movements are curtailed. During this month and next, a layer of fat is developing over the fetus’s entire body, which will enable it to adjust to varying temperatures outside the womb.

Nine Months after the formation of Zygote

About a week before birth, the fetus stops growing, having reached an average weight of about 7 ½ pounds and length about 20 inches, with boys tending to be little longer and heavier than girls. Fat pads continue to form; the organ systems are operating more efficiently, the heart rate increases, and more wastes are expelled through the umbilical cord.

The reddish color of the skin is fading. At birth, the fetus will have been in the womb for about 266 days, although gestational age is usually estimated at 280 days because most doctors date the pregnancy from the mother’s last menstrual period.

Multiple Births- Dizygotic

Multiple births occur in two ways. Most commonly, the mother’s body releases two ova within a short time (or sometimes, perhaps, a single unfertilized ovum splits), and then both are fertilized. The resulting babies are Dizygotic (two eggs) twins, commonly called fraternal twins.

The second way is for a single fertilized ovum to split into two. The babies that result from this cell division are monozygotic (one egg) twins, commonly called identical twins. Triplets, quadruplets, and other multiple births can result from either of these processes or a combination of both.

The risk in multiple births is due in part to a trend toward delayed childbearing since such births are more common among older women. Another critical factor is the increased use of fertility drugs, which spur ovulation, and of such techniques as in vitro fertilization. These trends are of concern since multiple births generally are more likely to be high-risk births.

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