Fig. 5 in Morphogenetic gradients in graptolites and bryozoans

Fig. 5. Diagram illustrating the introduction and spread of new thecal characters in monograptid colonies (A, B, D, E) with attempted biological interpretation of evolutionary changes involved (C, F). A, B. Proximal introduction and distalward spreading (as indicated by an arrow) of a phylogenetic novelty, interpreted (in C) as a result of increasing activity of a morphogen produced by the oozooid (siculozoooid) and acting as a stimulator of a given character, either due to increase of its amount (change from continuous oblique into broken oblique line) at the stable threshold level of the reactivity of the tissue, or due to increase of reactivity of tissues (lowering of the threshold level from t1 to t2) at the stable amount of morphogen produced. D, E. Distal introduction and spreading toward the proximal end (as indicated by an arrow) of a phylogenetic novelty, explained in F as a result of decreasing activity of a morphogen acting in this case as an inhibitor of a given character, either in result of its decreasing amount (change from continuous to broken oblique line) at the stable level of the reactivity of tissues (t1) or by the rise of the threshold level (t2) and decrease of their reactivity at the stable of morphogen produced by the oozooid. Note that the expressivity is indicated by the intensity of shading, while penetrance by numbers of zooids affected to any degree. From Urbanek (1960, 1973).