The Ectodermal Thickening and Ridge in the Limbs of Staged Human Embryos

T H E limbs of amniote embryos are characterized by a thickening of the epiblast at their tips (Balfour, 1885). Kolliker (1879) illustrated such a thickening in a section of a chick embryo and, according to Braus (1906), was responsible for the term 'ectodermal cap' {Ektodermkappe). Saunders (1948) showed in the chick by experimental means that 'the thickened ectoderm at the apex of the bud' is 'essential to the formation of the limb'. A similar thickening has frequently been noted at the apex of developing mammalian limbs. It can be seen in figures 3,4, and 5 of Lewis's (1902) paper on the development of the upper limb in man. Bardeen & Lewis (1901, fig. 19) called attention to the thickened epithelium at the free edge of the human lower limb bud. Steiner (1929, fig. 5), in a study of the development of human skin, illustrated the 'ectodermal cap' in the upper limb of a 9-mm. human embryo. He stated that it is several layers thick, that it is found mainly on the caudal and medial surfaces of the limb anlagen, and that it probably has a formative influence on mesoderm. The ectodermal ridge is present in a photomicrograph of a 10-mm. embryo published by Haines (1947, fig. 1). An 'epithelial plaque' at the apex of a 10-mm. human upper limb bud is figured by Hamilton, Boyd, & Mossman (1952, fig. 381). Fischel (1929) referred to the higher epithelium located at the distal end of the limb bud in many types of animals and termed Epithelfalte, Ektodermkappe, or Randfalte. He considered the term Epithelverdickung more appropriate in the human embryo and he showed this thickening at 6 mm. and at 9 mm. in two of his illustrations. He regarded the thickening as a growth centre for increase in