Is the Progress Zone Model a Victim of Progress? John W. Saunders  Cell  Volume 110, Issue 5, Pages 541-543 (September 2002) DOI: 10.1016/S0092-8674(02)00936-4

Figure 1 Skeletal Elements of the Wing of the Fowl, and Significant Signaling Systems Involved in the Execution of Their Pattern (A) Skeletal parts of the chick wing: h, humerus; r, radius; u, ulna. (B) Limb bud of chick embryo showing interactions (arrows) between products of genes expressed, as color coded, in the AER and the ZPA (source of SHH). Cell 2002 110, 541-543DOI: (10.1016/S0092-8674(02)00936-4)

Figure 2 Shoulder Girdle and Wing Skeletal Parts Formed after Excision of the AER at Successively Later Stages in Development of the Chick Wing Bud Although the shoulder girdle develops independently of the AER, its elements are shown for purposes of orientation: c, coracoid; s, scapula. Other elements identified by reference to Figure 1B. Cell 2002 110, 541-543DOI: (10.1016/S0092-8674(02)00936-4)

Figure 3 Representation of the Progress Zone Model Mesenchymal cells exit the PZ (arrows) by virtue of proliferation. Meanwhile, their proximodistal positional character has been specified by the length of time spent under the influence from signals from the AER and ZPA. Those exiting first show the axial configuration of stylopodial elements; cells for zeugopodial and autopodial elements presumably exit subsequently, having their axial character fixed in that order. Cell 2002 110, 541-543DOI: (10.1016/S0092-8674(02)00936-4)