the hen's egg test on chorioallantoic membrane, or hen's egg test is a test that can be used to test the toxicity of substances. it is also often used to test vaccines. it is a cam assay. the product ot be tested is applied to a special membrane. such tests were first used because chicken eggs were easily available. as the respective membrane does not contain any nerve cells, it is likely that the developing bird embryo does not feel pein during the test. these tests are not considered to be animal testing.