562 GERMINAL MEMBRANE.—DEVELOPMENT OF POLYPES. - 
spreads itself out so as to come into contact with the “albu 
men” of the seed by its external surface, the “germina 
membrane” of the Animal forms itself around the yolk, anc 
thus constitutes as it were a temporary stomach, within whicl 
the nutrient material is stored-up, and through the walls o 
which it is drawn into the embryo. This is accomplished in 
the following manner. The spherules of the outer layer o 
the mulberry-mass which are in immediate contact with thi 
yolk-bag become invested with walls of their own, and thu 
become converted into proper cells ; these are somewhat flat 
tened and of a polygonal shape, very much resembling thos 
of the epithelium of serous membrane (fig, 10). Anothe: 
layer is afterwards formed within this, the cells of whicl 
retain more of their original globular form. But the spherule: 
of the internal portion of the mulberry-mass, instead of be 
coming converted into cells, undergo’ dissolution and returi 
to the condition of a liquid yolk ; so that the ovum, in thi 
stage of its development, consists of two layers of cells, con 
stituting what are known as the “serous” and the “ mucous’ 
layers of the germinal membrane, enclosing a mass of nutritiou 
matter on which a change has been worked that seems t 
predispose it to become organized. 
738. The development “of the Polypes seems to advance 
but little beyond this point. The covering of the ovun 
bursts, and the contained embryo is set at liberty as soon ai 
the germinal membrane has been formed around the yolk 
In this state it becomes clothed with cilia, and is termed : 
gemmule ; and it swims about freely in water for some time 
Its form gradually becomes more elongated (fig. 303) 
tapering away at one end, which attaches itself afte: 
a time to some solid body; and its developmen 
into the polype-form soon commences. In the grouy 
of which the Hydra is an example, this change take 
place in the following simple manner. The germina 
membrane gradually thins away at the point furthes 
removed from the attached base, and at last an aper 
ture is formed, which becomes the mouth; from around thi: 
aperture the tentacula or arms shoot forth, a single row bein; 
first formed, and others being afterwards added in those specie: 
in which they are numerous. Thus the two layers of th 
germinal membrane enter into the permanent structure o 
Fig. 303. 
