464 EMBRYOLOGY OF THE CHICK 



germ layers with them, we shall have represented the formation of the 

 head fold. By pushing under the cloth at the other end of the bottle, 

 in the same way, we may represent the formation of the tail fold; and 

 in a like manner the lateral folds may be formed. If these folds, the 

 head, tail, and lateral be pushed under far enough, they will meet under 

 the center of the bottle, and we shall have the bottle, with its surround- 

 ing layers of cloth, connected with the rest of the model by only a sort 

 of stalk, which is hollow and composed of the three layers of cloth. The 

 bottle is used simply to give a solid object around which the folding 

 may be more easily done, but we are to consider the space occupied by 

 the bottle as an empty space. 



"We have now represented what is sometimes called the embryo- 

 sac, or simply the embryo, in contradistinction to the yolk-sac, or simply 

 the yolk. The embryo remains connected with the yolk throughout the 

 period of incubation by the yolk- or somatic-stalk, and as the embryo 

 increases in size, the yolk-sac is, by absorption, constantly diminished. 

 The space occupied by the bottle, in our model, represents the digestive 

 tract of the chick, and is lined, as will be seen by examination of the 

 model, by the lower germ layer, or entoblast. The body cavity would 

 be difficult to represent in the cloth model, but it can be imagined to 

 exist as the narrow space between the two layers of similarly colored 

 cloth which we have just called the mesoblast. 



"The formation of the amnion may be represented in our model by 

 lifting up with the fingers a small fold of the upper and second layers of 

 cloth, and pulling these two layers back over the head end of the embryo, 

 this fold will correspond to the head fold of the amnion. Similar folds 

 might be lifted up at the posterior end and at the sides of the embryo 

 model, to represent the tail and lateral folds of the amnion. The way in 

 which these folds fuse together will be explained later." 



The allantois cannot be explained from the model, but can be un- 

 derstood by studying Figure 270. It arises as a thin-walled pouch from 

 the posterior end of the digestive tract, and as it increases in size, it ex- 

 tends around the upper side of the embryo, between the inner and outer 

 layers of the amnion. 



Both amnion and allantois are thrown off at hatching, so take no 

 permanent part in the actual embryo. 



