Early development of the cruciate ligaments in staged human embryos.

Investigations were carried out on 43 serially sectioned human embryos of developmental stages 18 to 23. The homogeneous interzone of the future knee joint is observed in embryos at stage 18. During stage 19 this interzone is differentiated into dense, intensively stained, peripheral parts, which are the primordia of menisci and the medial portion, in which the cruciate ligaments are formed. All structures of the interior of the knee joint are more clearly delineated during stage 20, and they are well developed during the last embryonic week (stages 21-23).