 There are various weaknesses associated with normal stages that accompany the fulfillment of these investigative and explanatory goals. Key morphological indicators sometimes overlap stages, terminology that is useful for one purpose may be misleading for another, particular terms can be misleading in cross-species comparisons, and manipulation of the embryo for continued observation can have a causal impact on ontogeny. Avoiding variability in stage indicators can encourage overlooking the significance of this variation, or at least provide a reason to favor its minimization.