ABSTRACT
Serial sections of the limbs of seventy human embryos, ranging from developmental horizons 11 to 19 inclusive, were examined.
In the upper limb an ectodermal thickening was present on the ventral aspect and lateral border at horizon 12. The lateral part of this thickening formed the ectodermal ridge at horizon 14. The ridge disappeared during horizon 17. These events occurred one horizon later for the similar thickenings and ridges on the lower limb.
Changes in appearance and position of the limbs as seen in plaster reconstructions between horizons 12 and 17 are described. The limits of the thickenings and ridges were mapped out on these reconstructions and are illustrated.
The significance of similar ridges in avian limbs is discussed.
Although the term ‘ectodermal cap’ has been used frequently, presumably because of the appearance seen in microscopical sections, the term ‘ectodermal ridge’ is preferable, since it is topographically accurate (see Text-fig. 1).