Fig. 4. Analyses of mezzo expression in various mutants of the Nodal signalling pathway. All the embryos were fixed at the shield stage except those in B,H (40% epiboly). (A-F) Side views, (G-L) animal pole views. (A,G) Control wild-type embryos. Expression of mezzo was lost on the dorsal side in Zoep (B,H) and squint (C,I) mutant embryos. Expression of mezzo was abolished in cyc;sqt double mutants (D,J) and variably affected in MZsur embryos. The variable expression of mezzo in MZsur correlated with the expressivity of the schmalspur phenotype. The embryos shown in E,K derived from a cross producing severely affected embryos. Expression of mezzo is greatly diminished in two thirds of the marginal region in bon;faust double mutants (F,L).