16 Dynamic Theory. 



globe of liquid and forming its shell, on the interior of which shell an- 

 other layer of cells had formed itself, covering, however, but a small 

 part of the whole of the interior surface of the outer layer. The spot 

 thus formed is called the germ area or germ disc. The outer layer of 

 cells or shell is called the exoderm and the inner circular spot is called 

 the entoderm; the two terms meaning, respectively, the "outer skin" and 

 the "inner skin. " These two layers are designated as the "primary 

 germ layers," because the whole development of the animal proceeds 

 from them. The outside one, or exoderm, is also called the skin layer 

 and the animal layer. 



The inside one, or entoderm, is also called the intestinal layer, and 

 the vegetative layer. There is now formed between the entoderm and 

 the exoderm a third layer called the mesoderm or ' 'middle skin. " This 

 is somewhat smaller in area than the entoderm. It is supposed to be 

 formed by cells from both the entoderm and exoderm. At any rate it 

 soon becomes two layers instead of one, and the germ area or germ disc 

 now consists of 4 cellular skins which are distinct from one another and 

 could be split apart. They are now called respectively, beginning at 

 the outside, 



1. Skin sensory layer or skin stratum. 



2. Skin fibrous layer or flesh stratum. 



3. Intestinal fibrous layer or vascular stratum. 



4. Intestinal glandular layer or mucous stratum. 



In the meantime the entoderm has been increasing its area and rapidly 

 extending its cells in a single layer in all directions from the germ disc 

 until it entirely covers the inner surface of the exoderm, so that except 

 at the space called the germ disc, the ball of liquid is covered by a 

 double skin, the original exoderm, and the entoderm. 



During the above described formation of the four plates and before 

 the division of the middle one, other important changes are going on. 



The germ disc has changed in shape from circular first to an elliptic 

 form, then to an oval, and lastly, to the shape of a shoe sole or a fiddle. 

 That is, the oval has lengthened and the sides remain relatively con- 

 tracted making it narrower in the middle than at either end, but leaving 

 one end wider than the other. It is now called the germ shield on ac- 

 count of its changed form. The middle part of the shield becomes 

 much thicker and the plates become more distinct, especially the first 

 and fourth. The shield arches up in the middle and appears to rise 

 above the central part of the egg. A groove or trench is formed length- 

 wise through the center of the shield. On each side of this trench the 

 outside layer or skin stratum folds or doubles up and forms a low ridge. 

 These ridges continue to increase in height and incline toward each 

 other until they finally touch each other over the trench where they 



