562 GERMINAL MEMBRANE. DEVELOPMENT OP 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 cells, 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 " 

 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 

 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 



