Embryologic fusion planes and the spread of cutaneous carcinoma: a review and reassessment. It has long been held that embryologic fusion planes influence the spread of skin cancer. Embryologic fusion planes have been implicated in the depth of invasion, horizontal spread, and recurrence of cutaneous carcinoma. However, these structures have never been studied in detail. A review of the surgical literature reveals considerable confusion regarding the exact nature, location, and tumor interactions of these fusion planes. We review the gross and microscopic development of sites of embryologic fusion. We examine histologic sections through fusion sites in normally developed adult and fetal fresh cadaver specimens. Our studies, supported by our review of developmental anatomy, indicate that fusion planes do not persist as identifiable anatomic structures that would influence tumor spread. 