The pharyngeal pouches of the dog

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Abstract
The four pharyngeal pouches, pharyngeal grooves, and closing membranes of the dog form in an antero‐posterior sequence but the encroachment of head mesoderm which results in obliteration of the closing membranes occurs in a posteroanterior sequence. No pharyngeal clefts are formed. The anlage of the auditory tube arises as a dorso‐lateral extension from the dorsal surface of the first pharyngeal pouch and expands into an elongated tube directed toward the developing ear. The second pharyngeal pouch develops into a distinct tonsillar sinus dorso‐lateral to the base of the tongue. The palatine tonsil extends into the tonsillar sinus, and both become covered by a fold of epithelium. The third pharyngeal pouch extends medially toward the developing thyroid after contacting the cervical sinus. The distal tip of the third pouch differentiates into Complex III which transforms into parathyroid III and thymus III. The distal tip of the fourth pharyngeal pouch, Complex IV, also extends in a medial direction after contacting the cervical sinus. Complex IV transforms into parathyroid IV and the ultimobranchial body.