Counts have been made of the number of cells in individual laminae of the cortex in area 17 of the macaque monkey and cat, and degenerating terminals of geniculo-cortical axons have been studied with the electron-microscope in laminae IVab and IVc of the cortex of area 17 and lamina IV of area 18 of the visual cortex of the cat. The increase in number of cells through the depth of cortex in area 17 of the monkey compared with the cat would appear to involve about equally all laminae. In lamina IVab of area 17 of the cat the axon terminals of Y-type geniculo-cortical fibres make many more synapses per bouton than those of X-type fibres ending in lamina IVc. The geniculo-cortical axons ending in lamina IV of area 18 also make more multisynaptic boutons than those in lamina IVc of area 17. Thus, the terminals in the cortex of the X and Y functional types of fibre show comparable differences in both the cat and monkey.