When stimulated by a filterable agent of leukosis of chickens (Strain 2), endothelium may undergo seemingly unrestricted growth. These neoplasms of endothelium are usually unaccompanied by the formation of blood cells. Occasionally they produce hemocytoblasts, discharged like those of the normal marrow into vascular channels as also myelocytes about the vessels.

The same agent that stimulates endothelium also stimulates erythroblasts, myelocytes, and hemocytoblasts to unrestricted growth without obviously affecting the endothelium; and the association of endothelioma and leukosis is the result of stimulation of several types of cells by a single virus. Myelocytes appear also to develop from mesenchymal or endothelial cells without the intermediary stage of hemocytoblasts.

It is often impossible to determine whether the neoplasms caused by the virus of Strain 2 are of endothelial or mesenchymal origin, and it is possible that both types of cells may be stimulated by the same virus. Types of sarcoma like those described by Rous are not produced by the virus of Strain 2.

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