Sean B. Carroll, "Endless Forms Most Beautiful"

"All organizers share the property of influencing the formation of pattern, or morphogenesis, in tissues or cells. The basic interpretation of their special activity is that the cells of organizers produce substances that can influence the development of other cells. Such substances have been dubbed morphogens. The effects of organizers depend upon their distance from target cells...The idea then is that cells surrounding the source respond according to the amount of morphogen they experience...The morphogens responsible for the activity of organizers were some of the most sought "Holy Grails" of embryology The major difficulty that retarded further advances was that the organizer activity was a property of collections of cells. Any cell makes thousands of substances and it was always possible that more than one substance was responsible for organizer activity....They would wait decades."