GERMINAL MEMBRANE. 59 
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 in- 
ternal 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 per- 
ceived, in consequence of the dark substance lying beneath it. 
The cells also contain a transparent fluid, and some minute gra- 
nules with molecular motion, which is evidence sufficient for 
the existence of a peculiar cell-membrane. If, after the ger- 
minal 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 nu- 
cleoli. They also present many minute granules, which ex- 
hibit 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 epithe- 
lium 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 re- 
mains 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 trans- 
parent substance in which they le is composed of the cells, and 
is gelatinous only in appearance. The cells have only a mi- 
nimum of intercellular substance between them. 
When, in the next place, we proceed to examine the mucous 
