Period of The Zygote
The period of the zygote begins at fertilization. After a female egg is fertilized, the resulting one celled organism becomes known as a zygote. Once the egg is fertilized, the zygote begins a two-week period of rapid cell division and will eventually become an embryo.
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The zygote divides through a process known as mitosis, in which each cell doubles by dividing into two cells. This two-week stage is known as the germinal period of development and covers the time of conception to implantation of the embryo in the uterus. In most cases, each male and female sex cell contains 23 chromosomes.
When these two haploid cells join, they form a single diploid cell that contains a total of 46 chromosomes. The zygote begins a journey down the fallopian tube to the uterus where it must implant in the lining in order to obtain the nourishment it needs to grow and survive.
The period of the zygote lasts for about four days. Around the fifth day, 60 to 70 cells exist that form a hollow, fluid filled ball called a blastocyst. The embryonic disk will become the new organism; the outer ring will provide protective covering. Implantation occurs sometime between the seventh and ninth day which the blastocyst burrows deep into the lining of the uterus.
The amnion is a membrane that encloses the developing organism in amniotic fluid. The fluid functions as a cushion and temperature regulator. The yolk sac produces blood cells until the liver, spleen, and bone marrow mature enough to take over this function. The chorion, a protective membrane develops around the amnion by the end of the second week.
The placenta is a special organ that permits food and oxygen to reach the zygote and waste products to be carried away. The umbilical cord connects the placenta to the developing organism. The period of the embryo lasts from implantation through the eighth week of pregnancy.
The most rapid prenatal changes take place during these six weeks as the groundwork for all body structures and internal organs is begun. During the third week, the embryonic disk folds over to form three cell layers: 1.) Ectoderm- becomes the nervous system and skin.
2.) Mesoderm- from which will develop muscles, skeleton, cardiovascular system, and reproductive system. 3.) Endoderm- becomes the digestive system, lungs, urinary tract, and glands. The nervous system develops fastest in the beginning. The neural tube is a primitive spinal cord that forms when the ectoderm folds over. The embryo’s posture becomes more upright.
The embryo can move, and it responds to touch, especially in the mouth area and on the soles of the feet. The period of the fetus is the “growth and finishing” phase that lasts until the end of pregnancy. The third month the organs, muscles, and nervous system organize and connect. By the twelfth week, the external genitals are well-formed, and the sex of the fetus can be determined using ultrasound. Trimesters are three equal time periods in prenatal period, each of which lasts three months.
By the middle of the second trimester (which lasts from 13 to 24 weeks), the fetus has grown large enough that the mother cam feel its movements. Vernix is a white cheese like substance that covers the fetus and protects its skin from becoming chapped in the amniotic fluid. Lanugo is a white downy hair that also covers the fetus and helps vernix stick to the skin. At the end of the second trimester all the brain’s neurons have been produced.
The fetus can now be both stimulated and irritated by sounds and light. The third trimester is the age of viability, between 22 and 26 weeks, is the age at which the fetus can first survive if born early. The brain continues to make great strides during the last three months. The cerebral cortex enlarges and the fetus spends more time awake.
The fetus is also more responsive to external stimulation. The fetus moves less often, because of reduced space and greater ability to inhibit behavior. A layer of fat develops under the skin to assist with temperature regulation. In the last weeks, most fetuses move into an upside-down position.