Sentence:  He ... would begin to pick up the seed-cases or <WSD>mast</WSD>, squeeze each one with his fingers to see if it were fertile, and drop it if it were not. 
Meaning of the word marked with <WSD> tags:  The fruit of forest-trees (beech, oak, chestnut, pecan, etc.), especially if having fallen from the tree, used as fodder for pigs and other animals.