Fig. 1. The morphogenetic processes involved in Xenopus neurulation. All of the behaviors shown require proper establishment of either apicobasal or mediolateral cell polarity. During early neurulation, the neuroepithelium thickens apicobasally while convergent extension movements shape the neural plate. At mid-neurulation, convergent extension continues, while apical wedging initiates in the neuroepithelium, elevating the folds. Medially directed movement of the epidermis also begins. At late neurula stages, the neural fold fuse dorsally and convergent extension of the dorsal neural tissues shapes the neural tube. Neuroepithelium, blue; dorsal neural tissue and neural crest, yellow; epidermis, light blue; notochord, red. Adapted from Davidson and Keller (Davidson and Keller, 1999).