In [2] (page 27) Engelking states that:

The Niemytzki plane was defined (and attributed to Niemytzki), by Alexandroff and Hopf in [1]

which is accurate since in [1], where the plane is defined there is a footnote attributing that example to Niemytzki.

Does anyone know whether there is an article or book by Niemyzki where he defines the plane?

Bibliography:

[1] Alexandroff, P., Hopf, H., *Topologie I*, Berlin 1935.

[2] R. Engelking, *General Topology*, Heldermann Verlag, Berlin, Sigma Series
in Pure Mathematics 6, 1989.