There are two sources of enzymes for the midgut - the pancreas and the secretory cells in the gut wall - with the pancreas perhaps secreting the greater variety and quantities of enzymes in fish. Because of the variety of enzymes present in different species, there have been some attempts to correlate enzyme activities with diet. However, these enzyme studies are fragmentary and histochemical tests are too general. Much remains to be learned about intestinal digestion in fish.