"?-Catenin Defines Head Versus Tail Identity During Planarian Regeneration and Homeostasis" (2007), by Kyle A. Gurley, Jochen C. Rink, and Alejandro Sánchez Alvarado

Alejandro Sánchez Alvarado?s laboratory group has employed molecular tools to investigate old questions about regeneration and as a result have identified some of the molecular mechanisms determining polarity [4]. Recent work by his group has shown Wnt-?-catenin signaling determines whether a tail or a head will form during regeneration in planarians. This study was motivated by work Thomas Hunt Morgan [5] conducted in the late nineteenth century. Morgan observed that during regeneration a planarian cut into rather small pieces would sometimes regenerate a head at both the anterior and posterior ends of a pieace rather than a head and a tail. This led Morgan to think the size of the piece must affect the regenerative process.