Embryology
Embryology literally means the study of embryos, the immature organisms contained within the coverings of an egg or within the body of the mother. Embryology is a field of biology that focuses on the development of new individuals from germ cells.
Vertebrate embryology
Fertilization : The creation of the zygote
Instead of either of the male or female seed components containing a tiny baby, they are each just a cell. When they come together, the new cell, the fertilized egg - swells and changes. This zygote is literally the mother of all stem cells, as it gives rise to generations of cells, any one of which - if removed from the mass of multiplying cells in the early embryo, can itself give rise to all of the different kinds of tissues in the mature organism, muscle and brain, liver and lung, bone and bone marrow - in fact, if conditions are right, into a complete new individual organism. When twins are identical, they arose because the zygote or very early embryo became divided, and two rather than one, baby developed from a single zygote. Fraternal twins did not arise from the same mass of cells, but from two different egg and sperm combinations making two separate zygotes that develop together in the same uterus or womb. Identical twins are natural clones, where as fraternal twins are no more closely related than any two sisters, brothers, or brother and sister.
Gastrulation and the embryonic period
This zygote divides continually and the cells it becomes dance out a whole extravaganza of movements. The steps of this dance are known to all vertebrates. Whether frog or mouse, human or bird, the dance is nearly the same, right up until the basic form is created. As the cells swirl through their formations, each splits - and they all keep dividing, like the Sorcerer’s apprentice, until there are thousands and then millions and then billions of cells. The dance becomes an extravaganza with whole choruses of cells moving like dancers in a scene from a greatest ballet filmed in an MGM musical with an unlimited budget. Sheets of cells fold and flip to form an intricate tube. Troops of cells disappear to make a crevice, while another multiplying group piles into a bulge. Head to tail, the formations move to bring a shape to the vertebrate embryo.