Wasn't Dr Marco a sacrifice? This is kinda spoilers if you haven't watched all episodes, but my question is:

Wasn't Dr. Marco one of the few sacrifices? Envy didn't want Marco to die, but in the end wasn't used by Father. 
Or did I miss something? Was Marco only captured to create stones?
From the Full Metal Alchemist Wiki:


  Knowing this, the Crystal Alchemist attacks Envy using his hidden transmutation circle to critically damage the Homunculus' core stone. 


Knowing this, he likely has not seen the truth. And as such, would not have been a sacrifice, right away.

However, it's not unreasonable to think that the homunculi may have forced him to see the truth once they were ready, (similar to Mustang.)