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.