PATTERNS OF CONNECTIVITY IN THE NEOCORTEX OF THE ECHIDNA (TACHYGLOSSUS-ACULEATUS)

Patchy connections were traced in the visual and auditory cortex of th e echidna (Tachyglossus aculeatus). Labeled neurons and clusters of ax on collaterals were distributed in regular arrays after the applicatio n of a small crystal (similar to 100-300 mu m diameter) of the carbocy anine dye Dil (1,1'-dioctadecyl-3,3,3',3'-tetramethylindoc perchlorate ) into the upper cortical layers. In general, the anterograde and retr ograde labels were in register, hut 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 py ramidal 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 cor tices resembled that of eutherian mammals in containing a regular colu mnar array of connections that may represent the corticocortical proje ctions.