Guess the Cover of American Laboratory—October 2013

Correctly identify this month’s cover image and be entered to win a $10 Amazon gift card (one entry per person; you must be 18 years of age to be eligible. See contest rules at

Hint: Help us celebrate our 45th Anniversary with one of these, also a favorite with thoroughbred aficionados. Image captured using polarized light. Original magnification 600×.

What do you think it is? Enter by leaving your answer in the comments section below. Stay tuned for the answer and for next month’s cover image!