Dentsu CEO Tadashi Ishii and Aegis Group CEO Jerry Buhlmann

Dentsu CEO Tadashi Ishii and Aegis Group CEO Jerry Buhlmann

Daniel Lewis/VisualMedia via Bloomberg

Dentsu Inc. Chief Executive Officer Tadashi Ishii, left, and Aegis Group Plc Chief Executive Officer Jerry Buhlmann pose for a photograph ahead of the today's announcement in London.