PREVIOUS electron microscopic studies of chick embryo chorioallantoic membranes infected with herpes simplex virus have revealed that the development of virus in susceptible (ectodermal) cells apparently begins in the nucleus and is completed in the cytoplasm1. In the nucleus, the smallest particles considered to be virus measured 40–60 mµ in diameter and the largest did not exceed 130 mµ in diameter. Larger particles, some having a maximum linear dimension of 250 mµ, were seen in the cytoplasm or in extracellular locations. These observations were made on thin sections of tissue from which the methyl methacrylate had been dissolved by amyl acetate.