Histological sections through the head of developing axolotls at different stages, to show the degree of differentiation of the eye and especially the lens. At stage 36, there is only a lens vesicle (arrows) with no clear differentiation of lens fibers yet. At stage 39, cells at the posterior part of the lens vesicle start elongating (arrow) and differentiating to lens. This is the beginning of lens fibers differentiation. At stage 42, differentiation of lens fibers covering most of the lens is evident (red color, arrow). At stage 44 +1 the lens is fully differentiated with a lens epithelium in the anterior region and differentiated lens fibers at the posterior region. H & E staining.