Light microscopic findings of a median section of the posterior fossa of a chick embryo in chiari malformation with surgically induced open neural tube defect. Upper row, control group (left incubation day 17–18, right 20–21), lower rod, surgical group (left postoperative day 14–15, right 17–18). The subarachnoid spaces between the cerebellar cortex and the inner surface of the skull and between the cerebellum and the brainstem (marked with red arrow and yellow arrow, respectively) are narrower in the surgical group than in the control group. The cerebellum is elongated rostrally and caudally and located downward in the overcrowded smaller posterior fossa (hematoxylin and eosin staining, ×12.5).