1. Animal cell culture medium contains glucose, amino acids, growth promoting factors, inorganic salts and trace elements. It is also necessary to add natural substances such as serum and plasma. A is correct;

B, the primary cultured cells are not easy to be passed down to 10 generation, and the growth of cells is stagnant, and most cells are aging and dying. B is correct;

C, in animal cell culture, a few cells can survive to 40 ~ 50 generations, and cannot continue to be passaged after 50 generations, and the genetic material of some cells has changed, which has the characteristics of cancerous cells and can be passaged indefinitely under culture conditions, C error;

D, the principle of animal cell culture is cell proliferation, and a large number of healthy cells can be obtained for detecting toxic substances. D is correct.

So choose: C.