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 “germinal 
membrane ” of the Animal forms itself around the yolk, and 
thus constitutes as it were a temporary stomach, within which 
the nutrient material is stored-up, and through the walls of 
which it is drawn into the embryo. This is accomplished in 
the following manner. The spherules of the outer layer of 
the mulberry-mass which are in immediate contact with the 
yolk-bag become invested with walls of their own, and thus 
become converted into proper cells; these are somewhat flat¬ 
tened and of a polygonal shape, very much resembling those 
of the epithelium of serous membrane (fig. 10). Another 
layer is afterwards formed within this, the cells of which 
retain more of their original globular form. But the spherules 
of the internal portion of the mulberry-mass, instead of be¬ 
coming converted into ceils, undergo dissolution and return 
to the condition of a liquid yolk; so that the ovum, in this 
stage of its development, consists of two layers of cells, con¬ 
stituting what are known as the “ serous and the “ mucous 99 
layers of the germinal membrane , enclosing a mass of nutritious 
matter on which a change has been worked that seems to 
predispose it to become organized. 
738. The development of the Polypes seems to advance 
but little beyond this point. The covering of the ovum 
bursts, and the contained embryo is set at liberty as soon as 
the germinal membrane has been formed around the yolk. 
In this state it becomes clothed with cilia, and is termed a 
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 after 
a time to some solid body; and its development 
into the polype-form soon commences. In the group 
of which the Hydra is an example, this change takes 
place in the following simple manner. The germinal 
membrane gradually thins away at the point furthest 
Flg ’ 30w removed from the attached base, and at last an aper¬ 
ture is formed, which becomes the mouth; from around this 
aperture the tentacula or arms shoot forth, a single row being 
first formed, and others being afterwards added in those species 
in which they are numerous. Thus the two layers of the 
germinal membrane enter into the permanent structure of 
