In the excerpt below from Morphology of Invertebrate Neurons and Synapses
For the better part of a century, conclusions were made largely by matching the depth relations between the terminals of input neurons and the dendrites of their presumed targets; this practice became transferred to arthropods and cephalopods. The practice may be true for particular terminals or dendrites, and in vertebrates, but it has been long gainsaid by many examples in invertebrates. In the particular case of Drosophila, now known from dense reconstructions, EM denies any such simple dichotomy in important details, with many terminals having postsynaptic sites and many dendrites that are presynaptic as well as postsynaptic (Takemura et al., 2008).
I'm having trouble understanding what the acronym "EM" stands for, the author seemed to throw it in without prior definition (my guess is it's some neuroscience jargon). I also don't understand what the author means by "depth relation". Could someone explain to me the two terms assuming a college intro level background in neuroscience?