Light micrograph of a human blastocyst, an early stage of embryonic development, photographed 4-5 days after fertilisation. The blastocyst consists of a hollow ball of cells with a localized thickening called the inner cell mass (bottom left), which ultimately develops into the actual embryo. The remainder of the blastocyst (the thin part of the ball ) is composed of trophoblast tissue. Analysis of trophoblast cells has provided a novel means of pre-natal screening for genetic disorders. Initially unattached, the blastocyst soon becomes implanted in the wall of the uterus.