Which stage in development occurs first

The blastula is usually a spherical layer of cells the blastoderm surrounding a fluid-filled or yolk-filled cavity the blastocoel. Mammals at this stage form a structure called the blastocyst, characterized by an inner cell mass that is distinct from the surrounding blastula, shown in Figure During cleavage, the cells divide without an increase in mass; that is, one large single-celled zygote divides into multiple smaller cells.

Each cell within the blastula is called a blastomere. Cleavage can take place in two ways: holoblastic total cleavage or meroblastic partial cleavage. 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 and this mass of cells will go on to form the embryo.

At this stage of development, illustrated in Figure The trophoblast will contribute to the placenta and nourish the embryo. Visit the Virtual Human Embryo project at the Endowment for Human Development site to step through an interactive that shows the stages of embryo development, including micrographs and rotating 3-D images.

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. This process is called gastrulation. During gastrulation, the blastula folds upon itself to form the three layers of cells. Each of these layers is called a germ layer and each germ layer differentiates into different organ systems.

The three germs layers, shown in Figure 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. The endoderm gives rise to columnar cells found in the digestive system and many internal organs. It occurs in approximately monthly cycles throughout a woman's reproductive life, except during Release of the egg is called ovulation. The egg is swept into the funnel-shaped end of one of the fallopian tubes.

At ovulation, the mucus in the cervix becomes more fluid and more elastic, allowing sperm to enter the uterus rapidly. Within 5 minutes, sperm may move from the vagina, through the cervix into the uterus, and to the funnel-shaped end of a fallopian tube—the usual site of fertilization.

The cells lining the fallopian tube facilitate fertilization. If fertilization does not occur, the egg moves down the fallopian tube to the uterus, where it degenerates, and passes through the uterus with the next menstrual period. If a sperm penetrates the egg, fertilization results.

Tiny hairlike cilia lining the fallopian tube propel the fertilized egg zygote through the tube toward the uterus. The cells of the zygote divide repeatedly as the zygote moves down the fallopian tube to the uterus. The zygote enters the uterus in 3 to 5 days.

In the uterus, the cells continue to divide, becoming a hollow ball of cells called a blastocyst. The blastocyst implants in the wall of the uterus about 6 days after fertilization. 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. Feet and hands can be distinguished and now have fingers and toes digits , which may still be webbed.

The shell-shaped parts of the baby's ears are forming, and the baby's eyes are visible. The upper lip and nose have formed. The trunk of the baby's body is beginning to straighten. The beginnings of all key body parts are present, although they are not completely positioned in their final locations. Eyes, ears, arms and legs are identifiable. Fingers and toes are distinct and have nails. Use precise geolocation data. Select personalised content. Create a personalised content profile.

Measure ad performance. Select basic ads. Create a personalised ads profile. Select personalised ads. Apply market research to generate audience insights. Measure content performance. Develop and improve products. List of Partners vendors. While you might think of child development as something that begins during infancy, the prenatal period is also considered an important part of the developmental process. Prenatal development is a time of remarkable change that helps set the stage for future psychological development.

The brain develops over the course of the prenatal period, but it will continue to go through more changes during the early years of childhood. The process of prenatal development occurs in three main stages. The first two weeks after conception are known as the germinal stage, the third through the eighth week is known as the embryonic period, and the time from the ninth week until birth is known as the fetal period.

The germinal stage begins at conception when the sperm and egg cell unite in one of the two fallopian tubes. The fertilized egg is called a zygote. Just a few hours after conception, the single-celled zygote begins making a journey down the fallopian tube to the uterus. Cell division begins approximately 24 to 36 hours after conception. Through the process of mitosis, the zygote first divides into two cells, then into four, eight, sixteen, and so on.

A significant number of zygotes never progress past this early part of cell division, with as many as half of all zygotes surviving less than two weeks. Once the eight-cell point has been reached, the cells begin to differentiate and take on certain characteristics that will determine the type of cells they will eventually become. As the cells multiply, they will also separate into two distinctive masses: the outer cells will eventually become the placenta, while the inner cells form the embryo.

Cell division continues at a rapid rate during the approximately week-long journey from fallopian tube to uterus wall. The cells develop into what is known as a blastocyst. The blastocyst is made up of three layers, each of which develops into different structures in the body. Finally, the blastocyst arrives at the uterus and attaches to the uterine wall, a process known as implantation. Implantation occurs when the cells nestle into the uterine lining and rupture tiny blood vessels.

The connective web of blood vessels and membranes that form between them will provide nourishment for the developing being for the next nine months.