By May 1940 Terror Tales was nearing the end of its run (as were most shudder pulps), but it still had a great issue that begins with this cover by Rafael DeSoto and included some pretty classic pulp stories with some pretty classic pulp interior art, for example Russell Gray’s “Mistress of the Dark Pool.”
But most mad-sciency of all is “Test-Tube Frankenstein” by Wayne Robbins (1914-1958), a pretty good yarn about biological mimicry (and probably what inspired DeSoto’s cover, even if that latter isn’t really what’s going on there).
This issue of Terror Tales is available to read or download from the Internet Archive.