In three recent articles it was shown that β‐catenin is crucial for the establishment and the maintenance of the overall polarity and especially for the character ‘posterior’ in planarians. If the transcription of the β‐catenin gene was silenced by RNA interference, the overall polarity is lost, and in regenerating fragments a posterior blastema displays anterior characters by forming eyes and anterior ganglia. An attempt is made to integrate these new data, well‐known older observations, and observations from other regenerating systems into (...) an outline of a model that will clarify our current understanding as well as highlight areas still to be developed. (shrink)

The tacit standard view that development ends once reproductive capacity is acquired (reproductive boundary, or ‘‘RB,’’ thesis) has recently been challenged by biologists and philosophers of biology arguing that development continues until death (death boundary, or ‘‘DB,’’ thesis). The relevance of these two theses is difficult to assess because the fact that there is no precise definition of development makes the determination of its temporal boundaries problematic. Taking into account this difficulty, this article tries to develop a new species-dependent perspective (...) on temporal boundaries of development. This species-dependent account stands against both RB and DB theses since neither of them reflects the differences between species in the temporality of their development. In this perspective, I propose to use stem cells as a tool to analyze (1) the different developmental capacities of an organism during its life; and (2) the different developmental temporal capacities between species. In particular, I will show that stem cells enable four distinct temporal developmental patterns to be distinguished, i.e., four distinct temporal boundaries of development in the living. I show how these four patterns can be interpreted differently depending on the perspective one has on the definition of development. (shrink)