 He eventually hit upon another species of "Dioscorea", a Mexican yam known locally as "cabeza de negro", whose tubers were reported to grow up to 100 kg (220 lbs) in weight.As with much steroid chemistry, the Marker degradation relies heavily on kinetic control to degrade the sapogenin side chain while leaving similar functional groups on the steroid nucleus (relatively) unaffected.The first step is the reaction that led Marker to discover that the sapogenins had a ketone spiro acetal structure in their side chain.