Morphogenesis of the head of a newt: mesodermal segments, neuromeres, and distribution of neural crest.

Segmentation of the mesoderm in the head of a newt embryo is revealed by scanning electron microscopy. By the end of gastrulation, the newt embryo is already segmented from one end to the other, with additional segments added later by the tail bud. This metameric segmentation appears long before the first "somite" can be seen in the late neurula by light microscopy. The six segments found in the newt head look much like the six most-cranial segments described decades ago in shark embryos. Mesodermal segments in the newt head are similar to somitomeres in amniote embryos, but in amniote embryos, the numbers and relationships of head segments are quite different from those of the newt. In both amniote and newt, the first segment abuts the prosencephalon, but for each more caudal head segment, where the newt embryo has one segment, the amniote has two. Although the pattern and distribution of cranial neural crest is quite similar in newt and amniote embryos, there are different relationships between migrating crest masses and mesodermal segments due to the doubling of most of the cranial segments in amniotes. It now appears that all vertebrate embryos, regardless of their mode of gastrulation, form similar mesodermal segments from one end of the embryo to the other, and this metameric pattern is established during gastrulation.