THE AMNION. 245 



«„_T1ifi Amtiimi. 



Th e amnion is a fold of the somatopleure which rises tip as 

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

 ing over its back, fo rms a thin double ni eTnhra.ne between t he 

 embrYO an d the egg-sbelL Though a very characteristic struc- 

 ture, it i s^ of only secondary importa nce, and gi ves rise to no pa rt 



of flip- pmbryn itsplf 



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 tbcthirty-sixth hour 

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

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



Th is first formed ipart. or head fold, o f the am nion consists a t 

 first of epiblast oq Iy, i nasmuch as it arises from the proamnion , 

 or part of the blastode rm i mmediately in front of the embryo , 

 into which the mefoblast has no t yet spread (c/. 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 raj)idly, 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 s imilar tail fold is formed dur ing the second day ; and, 

 a li ttle later, side folds ap pear, connecting the head tod tkil folds 

 togethe r. The embryo is now completely surrounded by the 

 a mnion^ w hic h forms a low wall round its siTIes an d tan, and ex- 

 tends backwards over the head and neck as a thin membrn.nnns 



_ca2^ 



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

 ((/. Fig. 129, an) consist from the first of both epiblast and 

 mesoblast ; i.e. are folds of the somatopleure, "tTeyond 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, an') 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- 

 pletelj' covered by the amnion. As the amnion folds meet, 

 th ey coalesce, t he i nner layers of the folds forming a continu ous 

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



