THE A3INIOX. 245 



8. The Amnion. 



The amnion is a fold of the somatopleure which rises up as 

 a wall all round the embryo, a little distance from it, and, spread- 

 ing over its back, forms a thin double membrane between the 

 embryo and the egg-shell. Though a very characteristic struc- 

 ture, it is of only secondary importance, and gives rise to no part 

 of the embryo itself. 



The first trace of the amnion appears about the thirty-third 

 hour, as a small crescentic fold immediately in front of the head 

 of the embryo. This grows rapidly, and by the thirty-sixth hour 

 (Figs. Ill, 112, AX), has extended back over the anterior end of the 

 head as a transparent cap, formed by a double membranous fold. 



This first formed part, or head fold, of the amnion consists at 

 first of epiblast only, inasmuch as it arises from the proamnion, 

 or part of the blastoderm immediately in front of the embryo, 

 into which the mesoblast has not yet spread (cf. Fig. 109). 

 During the latter part of the second, and the third day, the 

 mesoblast gradually grows in from the sides, forming a thin 

 lining to the amnion, which from this time is two-layered. 



The head fold of the amnion extends backwards rapidly, and 

 before the end of the second day covers over the whole of the 

 head and neck region of the embryo. At the hinder end of the 

 embryo a similar tail fold is formed during the second day ; and, 

 a little later, side folds appear, connecting the head and tail folds 

 together. The embryo is now completely surrounded by the 

 amnion, which forms a low wall round its sides and tail, and ex- 

 tends backwards over the head and neck as a thin membranous 

 cap. 



Unlike the head fold, the side and tail folds of the amnion 

 (cf. Fig. 129, AX) consist from the first of both epiblast and 

 mesoblast ; i.e. are folds of the somatopleure, beyond the margin 

 of the embryo. 



During the third day the amnion grows rapidly on all sides, 

 and by the close of the day (Fig. 114, AN, AX') has covered 

 over the whole of the embryo, except a small patch near the 

 hinder end. During the fourth day the side folds meet each 

 other over the back of the embryo, which thus becomes com- 

 pletely covered by the amnion. As the amnion folds meet, 

 they coalesce, the inner layers of the folds forming a continuous 

 membrane, the inner or true amnion (Fig. 100, AX), which 



