An apparatus for facilitating fusion of adjacent vertebrae includes an implant body dimensioned for positioning within an intervertebral space between upper and lower vertebrae to maintain the vertebrae in desired spaced relation to facilitate fusion thereof. The implant body includes lower and upper surfaces for engaging the respective lower and upper vertebrae and first and second side wall portions extending between the upper and lower surfaces. The first and second side wall portions are substantially solid. At least one of the first and second side wall portions have a substantially narrow longitudinal slit defined therein arranged to enhance flexibility of the one side wall portion. Preferably, each of the first and second side wall portions includes a longitudinal slit. The implant body may define an internal chamber dimensioned for reception of bone growth inducing substances. The implant body includes an internal bore extending through the upper and lower surfaces for reception of bone growth inducing substances. Preferably, the internal bore is cylindrical shaped to receive a cylindrical bone dowel harvested during the surgical procedure, or alternatively, to receive a prepackaged cylindrical rod containing bone growth inducing substances. A plurality of internal bores may be provided in side by side relation.