sentences:  Although studies of the mechanisms of pioneer axons have mostly been in invertebrate models, studies have also begun exploring the role of pioneer axons in the development of large vertebrate axon tracts.The primary model for these experiments has been in the zebrafish.Like in "Drosophila", there is evidence to show that although pioneer axons play an important role in guiding the growth cones of follower axons, they may not be completely essential.