The type of cleavage depends on the amount of yolk in the eggs. Other species, such as birds, with a lot of yolk in the egg to nourish the embryo during development, undergo meroblastic cleavage. In mammals, the blastula forms the blastocyst in the next stage of development. Here the cells in the blastula arrange themselves in two layers: the inner cell mass and an outer layer called the trophoblast. The inner cell mass is also known as the embryoblast; this mass of cells will go on to form the embryo.
At this stage of development, the inner cell mass consists of embryonic stem cells that will differentiate into the different cell types needed by the organism.
The trophoblast will contribute to the placenta and nourish the embryo. Formation of the blastocyst : The rearrangement of the cells in the mammalian blastula to two layers, the inner cell mass and the trophoblast, results in the formation of the blastocyst. The typical blastula is a ball of cells. The next stage in embryonic development is the formation of the body plan. The cells in the blastula rearrange themselves spatially to form three layers of cells in a process known as gastrulation.
During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer, which differentiate into different organ systems. Differentiation of germ layers : The three germ layers give rise to different cell types in the animal body: the ectoderm forms the nervous system and the outer layer of skin, the mesoderm gives rise to muscles and connective tissues, and the endoderm gives rise to the lining of the digestive system and other internal organs.
The three germs layers are the endoderm, the ectoderm, and the mesoderm. The ectoderm gives rise to the nervous system and the epidermis; the mesoderm gives rise to the muscle cells and connective tissue in the body; and the endoderm gives rise to columnar cells found in the digestive system and many internal organs. Privacy Policy. Skip to main content. Animal Reproduction and Development.
Search for:. The length and weight of babies at delivery varies greatly. Early pregnancy can be difficult on your mind and emotions. Researchers estimate that between 10 to 25 percent of all clinically recognized pregnancies end in miscarriage pregnancy loss before 20 weeks. The rest usually happen before week When you get a positive pregnancy test, call your doctor to set up your first prenatal appointment.
Most doctors see patients about every four weeks during the first and second trimesters of pregnancy. Your baby hits a lot of milestones and markers before their delivery date. Each stage is important in the overall pregnancy picture. As your baby continues to develop, try to focus your efforts on taking care of yourself, keeping up with your prenatal appointments, and connecting with the life growing inside you.
Weeks 28 through 40 bring the arrival of the third trimester. Understanding a pregnancy week by week can help you make informed decisions and prepare for the big changes that lie ahead. A new study finds that epidurals do not affect child development in their later years.
A fetal arrhythmia is an irregular heart rate — too fast, too slow, or otherwise outside the norm. It's often benign. Postpartum diarrhea after a C-section is normal. Sharing our experiences of pregnancy and infant loss can help us heal. Using breast milk for eczema is a popular home remedy. Health Conditions Discover Plan Connect. Parenthood Pregnancy Embryo vs. Fetus: Fetal Development Week-by-Week. Share on Pinterest.
If more than one egg is released and fertilized, the pregnancy involves more than one fetus, usually two twins. Because the genetic material in each egg and in each sperm is slightly different, each fertilized egg is different. The resulting twins are thus fraternal twins. Identical twins result when one fertilized egg separates into two embryos after it has begun to divide.
Because one egg was fertilized by one sperm, the genetic material in the two embryos is the same. Once a month, an egg is released from an ovary into a fallopian tube. After sexual intercourse, sperm move from the vagina through the cervix and uterus to the fallopian tubes, where one sperm fertilizes the egg.
The fertilized egg zygote divides repeatedly as it moves down the fallopian tube to the uterus. First, the zygote becomes a solid ball of cells. Then it becomes a hollow ball of cells called a blastocyst. Inside the uterus, the blastocyst implants in the wall of the uterus, where it develops into an embryo attached to a placenta and surrounded by fluid-filled membranes. About 6 days after fertilization, the blastocyst attaches to the lining of the uterus, usually near the top. This process, called implantation, is completed by day 9 or The wall of the blastocyst is one cell thick except in one area, where it is three to four cells thick.
The inner cells in the thickened area develop into the embryo, and the outer cells burrow into the wall of the uterus and develop into the placenta. The placenta produces several hormones that help maintain the pregnancy. For example, the placenta produces human chorionic gonadotropin, which prevents the ovaries from releasing eggs and stimulates the ovaries to produce estrogen and progesterone continuously.
The placenta also carries oxygen and nutrients from mother to fetus and waste materials from fetus to mother. Some of the cells from the placenta develop into an outer layer of membranes chorion around the developing blastocyst.
Other cells develop into an inner layer of membranes amnion , which form the amniotic sac. When the sac is formed by about day 10 to 12 , the blastocyst is considered an embryo.
The amniotic sac fills with a clear liquid amniotic fluid and expands to envelop the developing embryo, which floats within it. The next stage in development is the embryo, which develops within the amniotic sac, under the lining of the uterus on one side.
This stage is characterized by the formation of most internal organs and external body structures. Most organs begin to form about 3 weeks after fertilization, which equals 5 weeks of pregnancy because doctors date pregnancy from the first day of the woman's last menstrual period, which is typically 2 weeks before fertilization. At this time, the embryo elongates, first suggesting a human shape.
Shortly thereafter, the area that will become the brain and spinal cord neural tube begins to develop. The heart and major blood vessels begin to develop earlier—by about day The heart begins to pump fluid through blood vessels by day 20, and the first red blood cells appear the next day. Blood vessels continue to develop in the embryo and placenta. Almost all organs are completely formed by about 10 weeks after fertilization which equals 12 weeks of pregnancy.
The exceptions are the brain and spinal cord, which continue to form and develop throughout pregnancy.
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