1. Intracerebral transfer in chicks of fowl pox virus produces marked changes in its behavior when studied in the chorio-allantoic membrane of chick embryos and in the skin of baby chicks.

2. A great and persistent increase in virulence for epithelial cells, characterized by rapid necrosis instead of proliferation and hyperplasia, is acquired by the virus propagated intracerebrally.

3. An affinity for cells of mesodermal origin including endothelial cells of blood vessels, and an increase in affinity for entodermal cells is acquired by the virus propagated intracerebrally.

4. The intracerebral virus causes a uniform morphological change in all types of cells, in that the infected cells rapidly become spherical in shape, detached, and desquamated followed by necrosis.

5. One intracerebral passage is sufficient to produce this change in the virus.

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