For the crosslinking agent, preferably used are divinyl compounds copolymerizable with the monomer, for example, N,N???-methylenebis(meth)acrylamide, (poly)ethylene glycol(poly)methacrylates, as well as water-soluble compounds having at least two functional groups capable of reacting with a carboxylic acid, for example, polyglycidyl ethers such as ethylene glycol diglycidyl ether, and polyethylene