Further observations on the site of bone prospective areas in the chick embryo wing bud.

The portion of the pre-axial region of the wing bud anterior to the cranial boundary of the skeletogenous territory of the stylo-zeugopod, according to Stark and Searls' recent maps, was surgically isolated in chicken embryos of the stages 18-25, and autoplastically grafted to the dorsal surface of the hind-limb bud or to the trunk. In other embryos of the same stages the cranial half or the cranial two thirds of the pre-axial region mentioned were isolated and heterotopically implanted. The experimental results consistently showed that the isolate contained a variously large portion of the presumptive radius and a significant part of the proximal half of the prospective humerus. In fact, the skeletal parts mentioned did not develop in situ in the donor wing while they differentiated, in general, in the implantation site. In this regard, the present research confirms data from previous experiments showing that Stark and Searls' maps do not offer an accurate representation of the position and the cranio-caudal width of the presumptive areas of the skeletal pieces of the wing. Our findings seem also to indicate that the major (or longitudinal) axis of the future radius is arranged along the proximodistal axis of the wing bud throughout the developmental stages mentioned; instead, the major axis of the future humerus undergoes a gradual shift, being first (stages 18-20) arranged nearly parallel to the bud base and getting more and more parallel to the proximo-distal axis of the wing bud through stages 21-24. Besides, between stage 18 and 25 the cranio-caudal width of the presumptive skeletogenous territory of the zeugopod, and to a lesser extent of the stylopod, seems to undergo a slight relative reduction with respect to the cranio-caudal thickness of the whole wing bud; this might depend on a process of aggregation of the prospective skeletogenous cells into gradually denser and more defined precartilaginous blastemes.