Slide showing diagrammatic drawing of the 'Cross section through the head of the embryo of fig. 134.' Slide shows diagrammatic drawing of the 'Cross section through the head of the embryo of fig. 134.' The diagram is labelled with four headings which are explained in the title below image: 'Fig. 135. Cross section through the head of the embryo of fig. 134. On the one side the optic vesicles of the primary and secondary embryo (r.pr.oc. and l.sek.oc.) are close together, on the other side (l.pr.oc. and r.sek.oc.) they are fused. (After Spemann, 1931a.)' Late 19th to early 20th century