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The eyes of amphibians, both anurans and urodelans, have interested students of development since the beginning of the twentieth century. Spemann investigated the roles of the optic vesicle and surface ectoderm in lens formation (Spemann, 1903, 1905, 1907, 1912, 1938) before he began his better known studies on neural induction and the “organizer.” In the United States, W. Lewis (1904, 1907a, b) and Harrison (1920, 1929) also employed this system of lens induction in their work on dependent and independent development. Mangold (1931) wrote a very complete review of the experiments on amphibian eyes up to that time. These early investigations consisted of the following types of microsurgical experiments: 1. isolating the prospective lens-forming ectoderm by removing the eye-forming region of the neural plate or the optic vesicle, 2. combining optic vesicle and ectoderm of different prospective significance located in other regions of the head or flank, and 3. transplanting the prospective lens ectoderm to other parts of the body. Several different species were employed and these were found to differ in the ability of the prospective lens ectoderm to differentiate into a lens without an eyecup and in the capacity of the optic vesicle to induce a lens from foreign ectoderm. Additional investigations seemed to indicate that these properties varied inversely in different species and led Spemann (1938) to propose the hypothesis of “double assurance” in lens formation. This concept postulates two processes, each of which can lead to lens development under experimental conditions: 1. self-differentiation of prospective lens ectoderm, and 2. lens induction by the optic vesicle. In normal development, these two mechanisms reinforce each other. In a later section of this account, it will be seen how recent investigations have led to refinements in our knowledge of the interactions leading to the development of a lens in its proper relation to the eyecup. |
