Fertilization marks the beginning of a new, individual life. At fertilization, the DNA of a single sperm and ovum merge to create the genetic blueprint for a new human being. Once the DNA has recombined and the single-celled ovum begins to divide, it is called a zygote. The first two cells of the developing zygote already have a designated -- though not entirely certain -- function. One new cell will grow and divide towards becoming the embryo, while the other will eventually develop into the amnion, chorion, and placenta, the embryo's 'support system' (6.).
The zygote continues to divide, forming a mulberry-like cluster of cells called the morula (7.). As the two groups of cells continue to divide, they form a hollow ball, now called a blastocyst. The cells gather into two masses. One -- the upper cell mass in the picture at right -- is the pre-embryo. The lower cell mass will become the embryo's support system: the chorion, amnion, and placenta (10.). A ring of cells will form around on the outer edge of the blastocyst, called the trophoblast (7.).
The developing zygote spends its first days of development travelling down the fallopian tube. At about the 6th day after fertilization, the trophoblast begins to implant in the uterus (13.). After this point, the developing human is seperate from its supporting structures and is called an embryo.
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