CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY CUSTOM MEDICAL STOCK PHOTO / SCIENCE PHOTO LIBRARY
Human blastocyst. Illustration of a human embryo 7-8 days after fertilisation, known as a blasto- cyst. The blastocyst (at centre) consists of a hollow ball of cells. It has implanted on the wall of the uterus (light brown, lower left). It is not the outer ball of cells of the blastocyst, but a localised thickening of cells inside it (orange) which will eventually develop into a human embryo. The outer blastocyst cells (blue) can be seen growing and dividing at the site at which they have implanted on the uterus. It is these cells which, at this site, will form the placenta that serves to nourish the growing embryo.
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