Three events during mammalian embryogenesis were reexamined that traditionally have been considered to be migratory. Using reconstructions of serially sectioned rat and human embryos and computer imaging, this descriptive study reexamined the following: sclerotome formation from the somite, spinal ganglion formation from the neural crest, and thyroid, parathyroid, and thymus gland formations from pharyngeal endoderm. The aim was to determine if the positional changes of the developing structures could be explained by differential growth (i.e., changes in the size and shape of the embryo and its parts) rather than migration (i.e., structures moving from one region of the embryo to another). Embryos and many of their internal structures, especially the neural tube, rapidly change in size and shape during the period when migration is described, but these phenomena were not considered in past studies. Surface reference points were used in past investigations that also move as the embryo and its internal structures enlarge. Computer imaging makes it feasible for the first time to keep magnifications identical for each series of reconstructions, rendering each one equivalent to one frame of a motion picture film. Using this technique together with more centralized, less mobile reference points, the study concludes that the positional changes that occur during the three events can be explained by differential growth. Anat Rec (Part B: New Anat) 289B:53-63, 2006. (c) 2006 Wiley-Liss, Inc.