After the fertilization, the zygote formed passes down the oviduct and divides repeatedly to form small ball of cells. after 5 -6 days, these cells moves to the uterus and attaches to the wall of uterus. these mass cells are termed as embryo .. this growing embryo inside the uterus needs food and produces some nitrogenous wastes to be removed..these functions are performed by placenta. it looks like  a disc attached to the uterus.        
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