Slide showing two diagrams of the embryos of 'twins which have been produced by median constriction of the unsegmented egg.' Slide shows two diagrams (one tinted, one black and white) of the embryos of 'twins which have been produced by median constriction of the unsegmented egg.' Diagrams are labelled 'a' and 'b'; diagram 'a' is tinted in blue ink. Title below images reads: 'Fig. 16 a and b. Twins which have been produced by median constriction of the unsegmented egg. Embryo a, which received its nucleus later, is much younger than embryo b. The same phenomenon may be observed in Fig, 7 and Fig. 8. (After Spemann, 1928)'. Late 19th to early 20th century