Fig. 1.

(A) Morphospace potential for bird toe proportions. The x-axis shows the ratio of P2/P1; the y-axis is the ratio of P3/P1. Black rectangles indicate P1, dark gray rectangles indicate P2, and light gray rectangles indicate P3. The variations in the toe proportions of a variety of skeletal preparations of birds are shown. (B) In chick digits the number of hours required for a phalanx to develop from the proximal to the distal joint is related to the size of the initial (distal) condensation. (C) Density of proliferating cells (BrdU-incorporated cells) is relatively high in the distal-forming tip of digits, but once the distal joint is formed on a phalanx, proliferation is reduced. No significant differences in proliferation rate are found among formed phalanges within a digit. (D) The size of the initial condensation is correlated with size of the condensation when the final pattern is achieved (day 10) in digits III and IV in chicken embryos.