Abstract
The author observed the pathology produced by fowl pox virus that had been modified by intracerebral passage and found that there was a marked change in its behavior both in the chorioallantoic membrane of the chick embryo and in the skin of baby chicks. These changes consisted of a great and persistent increase in virulence for epithelial cells, characterized by rapid necrosis instead of proliferation and hyperplasia, an affinity for cells of mesodermal origin including endothelial cells of blood vessels, and an increase in affinity for entodermal cells. One intracerebral passage was sufficient to produce this change in the virus.