98. Although analysis distinguishes the two elements of the neuro-muscular system, assigning separate properties to the separate tissues, an interpretation of the phenomena demands a synthesis, so that a movement is to be conceived as always involving Sensibility, and a sensation as always involving Motility.[126] In like manner, although analysis distinguishes the various organs of the body, assigning separate functions to each, our interpretation demands their synthesis into an organism; and we have thus to explain how the _whole_ has different _parts_, and how these different parts are brought into unity. Embryology helps us to complete the fragmentary indications of Anatomy and Physiology. 99. Take a newly laid egg, weigh it carefully, then hatch it, and when the chick emerges, weigh both chick and shell: you will find that there has been no increase of weight. The semifluid contents have become transformed into bones, muscles, nerves, tendons, feathers, beak, and claws, all without increase of substance. There has been _differentiation_ of structure, nothing else. Oxygen has passed into it from without; carbonic acid has passed out of it. The molecular agitation of heat has been required for the rearrangements of the substance. Without oxygen there would have been no development. Without heat there would have been none. Had the shell been varnished, so as to prevent the due exchange of oxygen and carbonic acid, no chick would have been evolved. Had only one part of the shell been varnished, the embryo would have been deformed.