102. Messrs. Foster and Balfour, avoiding the controverted designations of serous, vascular, and mucous layers, or of sensorial, motor germinative, and glandular layers, employ designations which are independent of theoretic interpretation, and simply describe the position of the layers, namely, _epiblast_ for the upper, _mesoblast_ for the middle, and _hypoblast_ for the under layer. From the epiblast they derive the epidermis and central nervous system (or would even limit the latter to the central gray matter), together with some parts of the sense-organs. From the mesoblast, the muscles, nerves (and probably white matter of the centres), bones, connective tissue, and blood-vessels. From the hypoblast, the epithelial lining of the alimentary canal, trachea, bronchial tubes, as well as the liver, pancreas, etc.[128] Kölliker’s suggestion is much to the same effect, namely, that the three layers may be viewed as two epithelial layers, between which subsequently arises a third, the origin of nerves, muscles, bones, connective tissue, and vessels.[129] 103. The way in which the history may be epitomized is briefly this: There are two germinal membranes, respectively representing the Instrumental and Alimental Systems. Each membrane differentiates, by different appropriations of the yolk substance, into three primary layers, _epithelial_, _neural_, and _muscular_. In the epiblast, or upper membrane, these layers represent: 1°, the future epidermis with its derivatives--hair, feathers, nails, skin glands, and chromatophores; 2°, the future nervous tissue; 3°, the future muscular tissue.[130] (Bone, dermis, connective tissue, and blood-corpuscles are subsequent formations.) The hypoblast, or under membrane, in an inverted order presents a similar arrangement: 1°, the unstriped muscular tissue of viscera and vessels; 2°, the nervous tissue of the sympathetic system; 3°, the epithelial lining of the alimentary canal with its glands. Fundamentally alike as these two membranes are, they have specific differences; but in both we may represent to ourselves the _embryological unit_ constituted by an epithelial cell, a nerve-cell, and a muscle-cell. All the other cells and tissues are adjuncts, necessary, indeed, to the working of the vital mechanism, but subordinated to the higher organites.