Slide showing tinted diagram of a 'Cross-section through the embryo of Fig. 25b.' Slide shows tinted diagram of a 'Cross-section through the embryo of Fig. 25b.' i.e. the bombinator toad. The diagram is labelled 'Pron.' and 'Oc.' Title below image reads 'Cross-section through the embryo of Fig. 25b. Oc., optic cup, formed by the implant; Pron., pronephric funnel. (After Spemann, 1919.) ' The optic cup on the embryo is tinted in red ink. Late 19th to early 20th century