101. This is not precisely the course observable in other vertebrates; but in all, the germinal membrane, which lies like a watch-glass on the surface of the yolk, is recognizable as two distinct layers of very similar cells. What do these represent? They are the starting-points of the two great systems: Instrumental and Alimental. The one yields the dermal surface; the other the mucous membrane. Each follows an independent though analogous career. The yolk furnishes nutrient material to the germinal membrane, and so passes more or less directly into the tissues; but unlike the germinal membrane, it is not itself to any great extent the seat of generation by segmentation. There are two yolks: the yellow and the white (which must not be confounded with what is called the _white of egg_); and their disposition may be seen in the diagram (Fig. 14) copied from Foster and Balfour’s work. The importance of the white yolk is that it passes insensibly into a distinct layer of the germinal membrane, between the two primary layers.[127] Each of the three layers of the germinal membrane has its specific character assigned to it by embryologists, who, however, are not all in agreement. Some authorities regard the topmost layer as the origin of the nervous system, the epidermis, with hair, feathers, nails, horns, the cornea and lens of the eye, etc. To the middle layer are assigned the muscular and osseous systems, the sexual organs, etc. To the innermost layer, the alimentary canal, with liver, pancreas, gastric and enteric glands. Other authorities are in favor of two primary layers: one for the nervous, muscular, osseous, and dermal systems; the other for the viscera and unstriped muscles. Between these two layers, a third gradually forms, which is specially characterized as the vascular. [Illustration: Fig. 14.--_Diagrammatic section of an unincubated hen’s egg._ _bl_, blastoderm; _w y_, white yolk; _y y_, yellow yolk; _v t_, vitelline membrane; _x_ and _w_, layers of albumen; _ch l_, chalaza; _a ch_, air-chamber; _i s m_, internal layer of shell membrane; _s m_, external layer; _s_, shell. ]