Lumbar hemiveebra results due to the abnormal development of the following embryonic structure:

The ventromedial pa of the somites is called the sclerotome. The cells of the sclerotome migrate medially. They surround the neural tube and give rise to the veebral column and ribs. The veebral body may ossify from two primary centers which soon fuse. One of these pas may fail to develop, resulting in only half of the body being present which is known as hemiveebrae. It is usually associated with absence of the corresponding rib.The lateral pa of the somite is called the dermatome which give rise to the dermis of the skin and to subcutaneous tissue. The intermediate pa of the somite is the myotome. It gives rise to striated muscle. Also know: Common anomalies of veebral column:Sacralization of L5 (the veebral body is fused to the sacrum)Lumbarization of S1 (it functions as a sixth lumbar veebra)Spondylolysis (a bony defect develops between the pedicle and the lamina)Spondylolisthesis (the veebral body, pedicles, and superior facet joints slide anteriorly leaving the posterior elements behind--most commonly at L5). Ref: Human Embryology By Inderbir Singh, 8th Edition, Page 92, 119.