(A) Comparison of WT and Gas1^–/– skulls differentially stained for bone (red) and cartilage (blue) demonstrated the reduced size of the Gas1^–/– skull. (B) Norma basalis of WT and Gas1^–/– skulls. Blue arrows indicate the absence of optic pillars in the Gas1^–/– mouse; yellow arrows highlight the Gas1^–/– cleft palate; black arrow indicates fenestration of the Gas1^–/– neurocranial base; green arrows indicate dysmorphic ectotympanic rings; and white arrows denote absence of the hypoglossal foramen. (C and D) Magnified norma basalis views of the maxillary and premaxillary palate of WT (C) and 2 Gas1^–/– (D) neonates without fully cleft palates. Red arrows highlight the morphologic range of fused incisors in Gas1^–/– mice; green arrows indicate the range of midline hypoplasia in premaxillary palatal shelves and paraseptal cartilages; and yellow arrows show the developing maxillary palatal shelves. (E–L) Comparison of WT (E) and Gas1^–/– (F–L) middle ear skeletal elements demonstrating the range of dysmorphology evident in Gas1^–/– mice. Outlined are the styloid process (green), stapes (orange), incus (yellow), and malleus (purple) in WT mice. Green arrows indicate tympanohyal segments; pink arrows indicate stylohyal segments; purple arrows indicate the mallei; yellow arrows indicate the incuses; blue arrows highlight the gonials; orange arrows highlight the stapes; red arrows indicate the ectotympanics; black arrows indicates ectopic cartilage between the stapes, styloid process, and ectotympanic; and white arrows indicate hypoglossal foreman. Exemplified in K is the fusion of the stapes to the otic capsule. Original magnification, ×1.8 (C and D); ×2.3 (E and F); ×1.3 (G–L).