They are secreted into the extrapallial space by the mantle, which also secretes the glycoproteins, proteoglycans, polysaccharides and chitin that make up the organic shell matrix.[4] Insoluble proteins tend to be thought of as playing a more important/major role in crystallization control.[23] The organic matrix of shells tends to consist of β-chitin and silk fibroin.[5] Perlucin encourages carbonate deposition, and is found at the interface of the chitinous and aragonitic layer in some shells.[6] An acidic shell matrix appears to be essential to shell formation, in the cephalopods at least; the matrix in the non-mineralized squid gladius is basic.[26]