DNA is like that. So if you have a complex structure on one end of the spine, a surprisingly modest change might plop an analog at the other end. From there to real functionality, hind limbs changing size to bigger / smaller, and the like, often hinges on nigglish changes in parameters.
Development of nerves, muscles, ligaments, bones, etc. probably built the same way. A bone that grows longer usually “prompts” the surrounding structures to accommodate its size. Can you visualize how cumbersome it would be for these things to “find” a working arrangement blindly? Evolution solves that one very elegantly, by “stumbling across” ways for adjoining structures to co-ordinate their growth. There is no plan and no purpose – – – this is, for me the most enormous proof of Intelligent Design – to make a Universe where DNA can happen.