GERMINAL MEMBRANE
to be composed of cells, which project forwards in the form of half-spheres, (plate II, fig. 5). A nucleus of the characteristic form may be recognised in some of them. It lies upon the internal surface of the cell-wall, is round, and contains one or two nucleoli. In most instances, however, no nucleus can be seen, either because none is present, or because it lies upon the posterior side of the cell, in which position it cannot be perceived, in consequence of the dark substance lying beneath it. The cells also contain a transparent fluid, and some minute granules with molecular motion, which is evidence sufficient for the existence of a peculiar cell-membrane. If, after the germinal membrane has lain for a time in water, the mucous layer be washed off, the general surface of these cells may be observed. They are then seen to lie close together, and to flatten against one another to hexagonal forms, (see plate II, fig. 6). They contain a beautiful nucleus, which encloses one or two nucleoli. They also present many minute granules, which exhibit molecular motion. The cells may also be observed in the recent germinal membrane, especially on its margin, at which part it is more transparent, and there they project forward in the form of large segments of a sphere. These cells then re- present the serous layer of the germinal membrane,—which, therefore, consists of round cells (their polyedrical form being referrible solely to their lying so closely together), furnished on the inner surface of their wall with the characteristic nucleus, and containing a clear fluid, and some isolated smaller granules. They might be conceived to be a mere covering of epithelium to the serous layer. But if the serous layer be separated after the blood has formed, for example, in an egg which has undergone forty-eight hours’ incubation, the vascular layer remains lying immediately upon this stratum of cells. Valentin has already recognised these cell-nuclei, for he says, that each of these layers of the germinal membrane consists of a transparent vitreous jelly, but that they are to be distinguished by the corpuscles which they contain. (Entwicklungsgeschichte, page 287.) These corpuscles are the cell-nuclei, the transparent substance in which they he is composed of the cells, and is gelatinous only in appearance. The cells have only a minimum of intercellular substance between them.
When, in the next place, we proceed to examine the mucous
