 For some species, the number of somites may be used to determine the stage of embryonic development more reliably than the number of hours post-fertilization because rate of development can be affected by temperature or other environmental factors.The somites appear on both sides of the neural tube simultaneously.Experimental manipulation of the developing somites will not alter the rostral/caudal orientation of the somites, as the cell fates have been determined prior to somitogenesis.