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, th^ niitpr nr falsa a.TtyTiInn 

 (Fig. 100, Az), which lies close beneath the vitelline inembrane, 

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

 layer of somatopleure investing the^yolk-sac. 



The s pace between the inner o '- t'T*^ firrmjnn nnd tlip. embryo 

 is c alled thfe cavity of the am nion. 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, for ming a wa ter- 

 ba th in which the embry o f*"" move freely i n any directio n. 

 Duri ng the later stage s of i ncuba tion, muscle fibres are deve- 

 lops d^in the mesoblast of the amnion, which by their cnntra c- 

 t.i oTiR rnnV the pmbryo to and fro within the e gg. 



The space between the inner and outer layers of the amnion 

 (Fig. 1 GO AN and Az) is, from the mode of formation of the amnion 

 (Figs. 114, 129), corr feiuous with the coelomic spa ce • ^jiich lies 

 be tween the two layers of the mesoblas t, b oth within the em bryo 

 andin the extra- embry onic regio n of the blast oderm. By the 

 sixth day the splitting of the mesoblast (c/. 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 witliin 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 charact CTistic_ f eature 

 in the_^eselQB ment of the three higher groups oT Vertebrates — 

 Beptilp.s B Ms. and Mamma ls. These same three. groups are 

 also c haracterised by t he presence ^dur ing th e later stages of 

 development, of an allantois, w hich plays an important part in the 

 re spiration of the embry o, and, Jn mammals, in its nutri tion as 

 w£il^^ The two struct ures, amnion and allant ois, are associated 



