FRANCIS LEROY, BIOCOSMOS / SCIENCE PHOTO LIBRARY FRANCIS LEROY, BIOCOSMOS / SCIENCE PHOTO LIBRARY
Blastocyst implantation. Diagram of the blastocyst (lower right), a cellular stage that occurs during human (and all mammalian) reproduction, five days after fertilisation. It is the stage between the morula and the gastrula. During this stage, implantation (upper left) takes place on the wall of the uterus (endometrium, orange). The outer trophoblast layer (light grey) implants to form the placenta (purple). The inner cell mass (darker grey) will form the embryo. At right, the chorion (red), yolk sac (right, black), amniotic cavity (left, black), amnion (left, light blue), embryonic disk (right, light blue), and blood spaces (pink) are also shown.
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