Patterns of connectivity in the neocortex of the echidna (Tachyglossus aculeatus)

Patchy connections were traced in the visual and auditory cortex of the echidna (Tachyglossus aculeatus). Labeled neurons and clusters of axon collaterals were distributed in regular arrays after the application of a small crystal (approximately 100-300 microns diameter) of the carbocyanine dye Dil (1,1'-dioctadecyl-3,3,3',3'-tetramethylindocarbocyanine perchlorate) into the upper cortical layers. In general, the anterograde and retrograde labels were in register, but whereas the anterograde label was distributed throughout all six layers, the retrogradely filled neurons were absent from layer 1 and the highest density of labeled cells was in layers 5 and 6. The cells contained within the patches were all pyramidal or pyramid-like and contained long spines on their dendrites. Therefore, despite their unusual location within the lateral posterior cortex, the internal structure of the echidna visual and auditory cortices resembled that of eutherian mammals in containing a regular columnar array of connections that may represent the corticocortical projections.