Fig. 3. Early somite stage Mixl1^–/– embryos exhibit multiple developmental abnormalities. (A-C) Lateral view of a five-somite wild-type (+/+) embryo (A), and lateral (B) and dorsal (C) views of a three-somite Mixl1^–/– (–/–) littermate. The Mixl1^–/– embryos do not adopt the lordotic morphology of the wild-type embryo, develop no heart and form a large allantois. The asterisk indicates the midline tissue mass separating the somites (s). (D) A 9.0 dpc Mixl1^–/– embryo showing disorganization of the neural folds and the formation of a bulbous caudal protrusion (double asterisks). (E,F) Anterior (E) and lateral (F) views of 8.25 dpc wild-type and Mixl1^–/– conceptuses. Note the absence of clearly defined midline structures (ml) in the Mixl1^–/– embryo. The silhouette of collapsed head folds of the mutant is outlined by a broken line. (G-M) Histological sections of 8.25 dpc wild-type and Mixl1^–/– conceptuses, at planes indicated in F. Note the poorly developed neural folds and foregut portal in the mutant. A compact core of cells (ax, J) is found in the midline in place of the notochord. Abbreviations: allantois (al), blood islands (bi), foregut portal (fg), heart (h), notochord (nc), neural folds (nf) and paraxial mesoderm (pm). Scale bar for histological sections: 50 µm.