SEPARATION OF GERM-SHIELD. 311 



which effect this separation, a furrow-like depression is first 

 formed round the embryo-body on the upper surface of the 

 germ- vesicle, surrounding it like a trench ; round the out- 

 side of this trench a circular wall, or dike, is formed by 

 the elevation of the adjoining parts of the germ- vesicle 

 (Fig. 94, 2 ks). 



In order to get a clear and connected view of this 

 important process, we may compare the embryo to a 

 fortress surrounded by a moat and a wall. This moat, or 

 trench, consists of the outer part of the germ-area, and 

 ceases where the germ-area passes into the intestinal germ- 

 vesicle. The important process of fission in the middle 

 germ-layer which occasions the formation of the large 

 body-cavity, extends over the whole germ-area along the 

 periphery of the embryo. At first the extent of this 

 middle germ-layer is co-extensive with that of the germ- 

 area; the whole remaining part of the intestinal germ- 

 vesicle originally consisting only of the two original germ- 

 layers, the outer and the inner. Thus, over the extent 

 of the germ-area, the middle germ-layer splits into the two 

 layers which we knew as the outer skin-fibrous layer, 

 and the inner intestinal-fibrous layer. These two layers 

 separate widely, a clear fluid collecting between them 

 (Fig. 94, 3 am). The inner layer, the intestinal-fibrous 

 layer, remains lying on the inner layer of the intestinal germ- 

 vesicle (on the intestinal-glandular layer). The outer layer 

 the skin-fibrous layer, on the contrary, attaches itself closely 

 to the outer layer of the germ-area, to the skin-sensoiy layer, 

 and the two together rise up from the intestinal germ-vesicle. 

 From these two united outer layers, a connected membrane 

 now arises. This is the circular Avail, which continues to 



