<qa>  What is the purpose of the study?The purpose of the study is to explore the safety, tolerability and efficacy of Spheramine cultured human retinal pigment epithelial cells on microcarriers in Parkinsons Disease patients with advanced disease who have insufficient symptom control by optimum oral medication. Patients are randomized to receive Spheramine injections into both hemispheres or a sham surgical procedure in a ratio of 1:1. A three month pretreatment period must be completed prior to surgery. Time to endpoint is 24 months. 