Oh Yeah, Developmental Biology!

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When a cell reaches the blastocyst stage (approx 100-150 cells) embryonic stem cells can be removed from the inner cell mass. It is these stem cells that give rise to the whole embryo as they have near unlimited differential and prolific prowess. The trophblast cells, which surround the inner cell mass and the blastocyst cavity (blastocoele), later develops into the placenta. 
(picture via IVF-infertility.com, words via me)

When a cell reaches the blastocyst stage (approx 100-150 cells) embryonic stem cells can be removed from the inner cell mass. It is these stem cells that give rise to the whole embryo as they have near unlimited differential and prolific prowess. The trophblast cells, which surround the inner cell mass and the blastocyst cavity (blastocoele), later develops into the placenta. 

(picture via IVF-infertility.com, words via me)