M. longitudinalis superior of the tongue was examined histologically in early (15-week) fetal specimens (N = 28). Its points of attachment and relationships are described and compared with corresponding features in the adult. This study, in particular, clarifies the relationship of the two longitudinal muscles (superior and inferior) in the tip of the tongue as well as the demarcation between the superior longitudinal and hypglossal muscles in the root of the tongue.
Barnwell, Y., et al. (1978). The anatomy of the intrinsic musculature of the tongue in the early human fetus: Part I. M. Longitudinalis Superior, International Journal of Oral Myology, 4 (3), 5-8.