In the foregoing consideration of the formation of receptors of the skin it was assumed that certain common stimuli of the environment hammer out for themselves paths in the nerve-fibrils of the skin and by ceaseless repetition lay down not only the receptor, which may be called the _terminus a quo_, but also the afferent fibres which ultimately find their way into the grey matter of the cord and brain. That this is the initial stage of the construction of the higher nervous system can hardly be denied. But it carries the problem of the synthesis of the organism but a little way unless it be coincident with the construction of new reflex-arcs and their co-ordination into systems. Till this stage be reached in a rudimentary form the most cunning and exact adaptations and structures, or, as they may be broadly called effectors, will not advance the efficiency of the organism in the smallest degree. If the receptor be the _terminus a quo_ the effector is the _terminus ad quem_. This is so obvious that it may be waved aside as a truism not worth the notice of a zoologist concerned with the major problems of biology. It may seem to challenge in a highly speculative region and manner the labours of the biometrician and Mendelian, but, if fairly met it no more encroaches on their territory than do the labours of the engineers who invented the first and crudest chassis of a motor car upon the elaborate and brilliant ingenuity, taste and skill of the coachbuilders who turn out the “body” of a sumptuous Rolls-Royce of 1920. But the latter would never have “arrived” if the former had not made his slow and arduous trials and errors and final success. So here, as in many other subjects, a truism has its use. If the biometrician and Mendelian will only abstain from erecting notice-boards to proclaim “No thoroughfare here,” we shall not be put down as trespassers or poachers on their ground and may range at large in certain fields of speculation. Some Neural Phenomena. Among numerous phenomena of nervous reactions discovered by the research of physiologists certain have a close bearing on the formation of receptors, afferent fibres and reflex-arcs, especially those of Delay, Summation, Fatigue, Block or Resistance, Localization, Facilitation and Inhibition. Facilitation.