Slide showing two diagrammatic drawings of twin embryos which have resulted from frontal constriction of the fertilised egg. Slide shows two diagrammatic drawings, (one black and white, one tinted) of twin embryos which have resulted from frontal constriction of the fertilised egg. Drawings are labelled 'a' and 'b'; diagram 'b' is tinted in blue ink. Title below images reads: 'Fig. 18 a and b. "Twins" resulting from frontal constriction of the fertilised egg. The dorsal half develops into a well-proportioned embryo; the ventral half forms a round piece without exterior differentiation (After Spemann, unpublished.)' Late 19th to early 20th century