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 is Terror Tales for July 1940, cover artist unrecorded. The ISFDB entry for this issue is here. This version of the cover was blogged by me at Infernal Wonders, with its source in turn by the vanished blog “The Golden Age.”
This is Terror Tales for January-February 1940, cover artist unrecorded. The ISFDB entry for this issue is here. This version of the cover comes from (I think, there are the usual indexing issues) this picssr cover-art gallery.