Figure 7.  Branchial apparatus. Diagram illustrates the developing branchial apparatus. The branchial clefts (or grooves) are ectodermal lined, whereas the pharyngeal pouches are of endodermal origin. The branchial arches are mesodermal in nature. The second branchial arch begins to overgrow the third and fourth branchial arches to form the cervical sinus of His. In normal development, the sinus is obliterated when the second branchial arch merges with the epicardial ridge. (Reprinted, with permission, from reference 26.)