||Krnjević, K. "Chemical nature of synaptic transmission in vertebrates." Physiological Reviews pp.418-540 :(1974) 54.2 p.499 top paragraph
|| Colonnier M. Synaptic patterns on different cell types in the different laminae of the cat visual cortex. An electron microscope study. Brain Res. 1968 Jul9(2):268-87  Harris AJ, Kuffler SW, Dennis MJ. Differential chemosensitivity of synaptic and extrasynaptic areas on the neuronal surface membrane in parasympathetic neurons of the frog, tested by microapplication of acetylcholine. Proc R Soc Lond B Biol Sci. 1971 Apr 27 177(1049):541-53.PubMed ID4175993, 4396519
||Primary source  p.268 bottom paragraph: "The cats were perfused with fresh formalin (refs 23, 40) under artificial respiration and appropriate blocks of cortex were immersed in osmic acid, stained with uranyl acetate in the block (ref 39) and embedded in Araldite (ref 16). Sections measuring 1-2 mm in width and at least as long as the whole thickness of the cortex were placed on Robertson slit grids and orientated to permit a continuous, unobstructed view of the whole thickness of the cortex from layer I to the white matter. The sections were stained with lead citrate (refs 26, 39)."
||P.499 top paragraph: "Although the density of synapses may not be much greater on mammalian cortical neurons than on the frog’s parasympathetic neurons [probably less than 10 % of the surface area is actually covered by synapses (cf. primary sources)], when synapses are separated by distances of 5µm or less, the responses of the cell may be determined primarily by the high-sensitivity spots." Primary sources  and  studied cat and frog, respectively