The nature of spondylolysis is still a matter of considerable controversy where opinions oscillate between regarding the condition a stress fracture or due to congenital defect at the interarticttlar isthmus' The subject of spondylolysis was first recorded bY lambl (lBbB) and Neugebauer (lBBi) and since that time it has aroused the interest of obste"
tricians and orthopaedic surgeons as a cause of forward displacement of lumbar vertebrae (spondylolisthesis).
Unilaterat or bilarteral cases were reported to occur in 4.2% of lumbar vertebrae with a, greater frequency in males than in females (Roche and Rowe, 1951 a). Previous investigations dealt with description of post-mortem specimens or verte'brae removed during surgical operations in addition to examination of long series of radiograms on ver'tebral eolumns. The objective of the present work is to investigate further the nature  of spondylolYsis as seen in driecllumbar vertebrae.