Surface changes in the embryonic interdigital epithelium during the formation of the free digits: a comparative study in the chick and duck foot.

The formation of the free digits of the chick is accompanied by conspicuous surface changes of the interdigital ectoderm. These changes were much less pronounced or absent in the duck. As early as the interdigital grooves were detected in the chick, the morphological features of the ectodermal cells changed from a polygonal shape and flattened appearance to a rounded shape and bulging appearance. These changes were not present in the webbed foot of the duck. On the other hand the development of the interdigital commissures was accompanied by the formation of ectodermal ridges consisting of an accumulation of rounded cells which were in some cases in course of detachment to the amniotic cavity. These ridges were very prominent in all the interdigital commissures of the chick. In the duck they were less pronounced and were only present in the first and third commissure. From these results it is suggested that in addition to the well-known interdigital mesenchymal necrotic process (INZ) the ectodermal tissue of the interdigits might also be actively involved in the formation of free digits.