Somite segmentation in amphibian embryos: is there a transmitted control mechanism?

In all vertebrate embryos, somite segmentation begins in the occipital region and proceeds from here caudally with remarkable regularity. The segmentation pattern and the shape, size and number of somites formed are constant for each species, and it appears as if some quite complex mechanism must control their development. That this control mechanism must be versatile as well as precise is shown, for instance, by the fact that haploid newts have a larger number of cells per somite than the normal diploid, while polyploids have smaller numbers of somite cells than normal, so that the somite sizes are very nearly the same in all cases: the number of somites per embryo is also always the same as in the diploid (Fankhauser, 1945). Further, when mesoderm is either added to or excised from the somite region of the archenteron roof experimentally (Waddington & Deuchar, 1953), there is regulation so that the overall shape of the somites remains nearly normal, and again, their number per embryo is unchanged from that typical of the species.