Fig. 1. The Bmpr1a;Bmpr1b double-mutant mouse is holoprosencephalic. (A) The genetic cross that generates the double mutant. (B,C) E11.5 double-mutant and control littermates. Note the small eye in the mutant (arrowhead). (D-K) Cresyl Violet staining of E10.5 (D-G) and E11.5 (H-K) control and mutant telencephalons from rostral (D,E,H,I) and more posterior coronal sections (F,G,J,K). D' and E' are magnifications of the rostral midline (boxed) in D and E, respectively, showing a thick neuroepithelium in the mutant. F' and G' are magnifications of the dorsomedial regions (boxed) in F and G, respectively, showing no invagination in the mutant. Arrowheads (H,I) point to the rostral midline, which is thicker in the mutant (I). Arrows (J,K) indicate the missing dorsal midline in the mutant (K). bg, basal ganglia; cx, cerebral cortex; dm, dorsal midline; dt, dorsal telencephalon; me, facial mesoderm; rm, rostral midline; vm, ventral midline; vt, ventral telencephalon.