104. This conception may be compared with that of His in the division of Archiblast and Parablast assigned by him to the germ and accessory germ.[131] We can imagine, he says, the whole of the connective substances removed from the organism, and thus leave behind a scaffolding in which brain and spinal cord would be the axis, surrounded by muscles, glands, and epithelium, and nerves as connecting threads. All these parts stand in more or less direct relation to the nervous system. All are continuous. By a similar abstraction we can imagine this organic system removed, and leave behind the connected scaffolding which is formed from the accessory germ; but this latter has only _mechanical_ significance; the truly vital functions belong to the other system. 105. The researches of modern histologists have all converged towards the conclusion that the organs of Sense are modifications of the surface, with epithelial cells which on the one side are connected with terminal hairs, or other elements adapted to the reception of stimuli, and are connected on the other side through nerve-fibres with the perceptive centres. It has been shown that nerve-fibres often terminate in (or among) epithelial cells--sensory fibres at the surface, and motor-fibres in the glands.[132] Whether the fibres actually penetrate the substance of the cell, or not, is still disputed. Enough for our present purpose to understand that there is a _physiological_ connection between the two, and above all that sensory nerves are normally stimulated through some epithelial structure or other. [Illustration: Fig. 15.--_Transverse section of a Blastoderm incubated for eighteen hours._ The section passes through the medullary groove, _m e_. A, epiblast. B, mesoblast. C, hypoblast. _m f_, medullary fold, _c h_, notochord. ]