The development of segmentation in the cranial region of vertebrate embryos.

The purpose of this review is to show how the development of metameric pattern in the mesoderm of vertebrate embryos is related to cranial segmentation. Direct visual evidence is presented which shows that the segmental development of several vertebrate embryos is initiated as early as gastrulation. In the paraxial mesoderm of the head, somitomeres, identified in stereo with the SEM, accumulate as tandem units. Somitomeres remain contiguous in the cranial region, but separate from one another in the trunk region as they differentiate into somites. Altogether, seven segments comprise the cranial mesoderm of bird, mouse, and snapping turtle embryos. The development of neuromeres in the brain of these embryos is coincident with that of somitomeres, but it is the expansion of the neural segments into brain regions that first distorts the early somitomere-neuromere relationships. Even the paths that cranial neural crest take as they emigrate from the midline, are related to the segmental pattern in the mesoderm. It is likely that the debate over cranial segmentation in vertebrates that has spanned over 150 years in the literature, may be resolved as more embryos are examined and their early patterns of development are realized.