coming from the ipsilaterally projecting rgcs. other factors influencing ipsilateral rgc growth include the teneurin family, which are transmembrane adhesion proteins that use homophilic interactions to control guidance, and nogo, which is expressed by midline radial glia. the nogo receptor is only expressed by vtc rgcs. finally, other transcription factors seem to play a significant role in altering. for example, foxg1, also called brain-factor 1, and foxd1, also called brain factor 2, are winged-helix transcription factors that are expressed in the nasal and temporal optic cups and the optic vesicles begin to evaginate from the neural tube. these factors are also expressed in the ventral diencephalon, with foxd1 expressed near the chiasm, while foxg1 is expressed more rostrally. they appear to play a role in defining the ipsilateral projection by altering expression of zic2 and ephb1 receptor production. growth in the optic tract once out of the optic chiasm, rgcs will extend dorsocaudally along the ventral diencephalic surface making the optic tract, which will guide them to the superior colliculus and lateral geniculate nucleus in the mammals, or the tectum in lower vertebrates. sema3d seems to be promote growth, at least in the proximal optic tract, and cytoskeletal re-arrangements at the level of the growth cone appear to be significant. myelination in most mammals, the axons of retinal ganglion cells are not myelinated where they pass through the retina. however, the parts of axons that are beyond the retina, are myelinated. this myelination pattern is functionally explained by the relatively high opacity of myelin—myelinated axons passing over the retina would absorb some of the light before it reaches the photoreceptor layer, reducing the quality of vision. there are human eye diseases where this does, in fact, happen. in some vertebrates, such as the chicken, the ganglion cell axons are myelinated inside the retina.