MADERSON, PAUL F.A.; Brooklyn College of CUNY: Old theories die hard – but cells move only one way in growing feathers

Feather development and replacement are complex phenomena. Classical histological studies showed that apical tissues were first-formed and interpreted growth to involve proximo-distal movement of cells away from basal germinal locations. The Chicago School’s “Concrescence Theory (CSCT)”, usually attributed to Lillie and Juhn (1932), claimed that new barb loci arose on either side of the ventral mid-line of the collar and moved around the latter towards the rachidial ridge: seemingly this additional trajectory was proposed to explain certain of their experimental results. “Modifications” in the Chicago School’s later papers involved substantive changes in graphic representations, but not in language. ‘Espinasse (1939) provided point-by-point criticism of that body of work, and, alluding to Lillie and Juhn (1938), commented: – “It does not appear to be possible to discover for certain whether these authors are in this passage seeking to rehabilitate the theory of concrescence or not, but it is believed that they are.” Lillie’s (1942) widely quoted review does not speak directly of CSCT – the phrase is not used, but accompanying diagrams and text apparently ignore those criticisms. By the 1940s, figures, original and redrawn, and text wordings alluding to CSCT had become, and remain to this day, widely publicized in secondary and tertiary literature. Only Sengel (1976) discusses refuting experimental evidence and states explicitly that CSCT should be abandoned: there is no evidence that cells move other than in a proximo-distal direction. Nevertheless, Prum (1999) invokes barb loci movement around the collar as a morphogenetic mechanism involved in his developmental model for feather origin and evolutionary diversification. The fact that such movements do not occur in modern feathers weakens that model.