ADULT VS EMBRYO. “In adult mammals such as mice and humans, the wound heals and a scar is formed after injury, while wound healing is completed without scarring in an embryonic mouse. Completion of regeneration and wound healing takes a long time in regenerative and non-regenerative limbs, respectively.”
http://onlinelibrary.wiley.com/doi/10.1111/j.1440-169X.2007.00973.x/full