246 THE CHICK. 



closely invests the embryo, and is continuous with the margin 

 of its body wall (Fig. 129). The outer layers of the amnion 

 folds also form a continuous membrane, the outer or false amnion 

 (Fig. 100, az), which lies close beneath the vitelline membrane, 

 and soon fuses with this, while peripherally it passes into the 

 layer of somatopleure investing the yolk-sac. 



The space between the inner or true amnion and the embryo 

 is called the cavity of the amnion. It is filled with fluid, and is 

 at first very small, the true amnion on the fourth and fifth days 

 investing the embryo very closely (Fig. 100). . During the 

 following days, owing to accumulation of fluid within it, the 

 amnionic cavity increases very considerably, forming a water- 

 bath in which the embryo can move freely in any direction. 

 During the later stages of incubation, muscle fibres are deve- 

 loped in the mesoblast of the amnion, which by their contrac- 

 tions rock the embryo to and fro within the egg. 



The space between the inner and outer layers of the amnion 

 (Fig. 100 AN and az) is, from the mode of formation of the amnion 

 (Figs. 114, 129), continuous with the ccelomic space which lies 

 between the two layers of the mesoblast, both within the embryo 

 and in the extra- embryonic region of the blastoderm. By the 

 sixth day the splitting of the mesoblast (cf. Fig. 100) has 

 extended about half-way round the yolk-sac. The further exten- 

 sion of the splitting takes place much more slowly, and does 

 not reach the lower pole of the yolk-sac until within a few days 

 of the time of hatching. 



About the tenth day(c/. Fig. 101), when the splitting of the 

 mesoblast has extended about three-fourths of the way round 

 the yolk-sac, a circular fold of somatopleure arises from near 

 its* ventral edge, and grows over the dense mass of albumen, 

 WA, at the lower surface of the egg, inclosing this in much the 

 same way as the amnion incloses the embryo at an earlier stage, 

 and aiding in the absorption of this mass of albumen. 



The formation of an amnion is a very characteristic feature 

 in the development of the three higher groups of Vertebrates — 

 Reptiles, Birds, and Mammals. These same three groups are 

 also characterised by the presence, during the later stages of 

 development, of an allantois, which plays an important part in the 

 respiration of the embryo, and, in mammals, in its nutrition as 

 well. The two structures, amnion and allantois, are associated 



