ABSTRACT
Three types of microscopic operations were used in this study: 1. excision of the optic vesicle alone; 2. after excising the optic vesicle, a piece of forebrain tissue taken from another donor was inserted in between presumptive lens ectoderm and forebrain wall; 3. cut down the optic vesicle and portion of forebrain tissue and replaced them back in situ by turning over 180?. The operations were carried out on Rana nigromaculata at 15 and 16 stages, in order to bring the presumptive lens ectoderm to come into contact directly with the forebrain tissue for the purpose to analyse the possibility of eye cup formation from the brain wall. The results indicate that the forebrain wall which came into contact with the presumptive lens ectoderm could be induced and differentiated into a secondary eye cup or retina. A total of 81 cases of secondary eye cups among 282 operations (28.8%) were observed. The mechanism for the induction and its significance were discussed.