Vertebrate skull evolution: 5-Mammals
Mammalian skull, showing the reduced mandibular arch elements: mandibular arch (green - including two middle ear ossicles), hyoid arch (red - including stapes and ossicles connecting to hyoid and larynx), branchial arches (orange - including laryngeal cartilages)
Branchial arch and gill slit formation in vertebrates
Gill slits arise as the endoderm (green central tube) evaginates to connect to an ectodermal (outer brownish layer) invagination. In between two consecutive gill chambers, the branchial arches form: green: mandibular arch, red: hyoid arch, orange: branchial arches.