In 1926, Harvard physicians discovered that eating half a pound of liver daily would prevent Pernicious Anemia. The hunt was on to identify the compound in liver that was responsible. In 1947, Karl Folkers (a scientist at Merck) isolated the tiny red crystals of vitamin B12. 'Extrinsic Factor' now had a name.
It was first noticed in 1929 that giving beef alone to Pernicious Anemia patients did not improve the anemia, but when given with human gastric juice, it did. Giving gastric juice alone (without beef) didn't help either. In 1936, the scientist, W.B Castle called the substance in beef 'Extrinsic Factor' and the substance in gastric juice 'Intrinsic Factor'. They only work when given together. Today we know Extrinsic Factor by a different name, vitamin B12.