655 



AMNION AND ALLANTOIS 



The amnion is a peculiar membrane which envelops the embryo. 

 It arises in the following way. At a time when the splitting of the 

 mesoderm into somatic and splanchnic layers has progressed 

 some way outwards from the embryo over the yolk there appear, 

 starting in the proamnion, upward folds {a^n. in Fig. 503, 5 ; 

 a.fld. in Fig. 508) parallel with the downward folds which formed 

 the embryo, but consisting of somatopleure only. The folds on 

 all sides of the embryo arch upwards and unite above, forming 

 a dome over the embryo. When their tops unite, the inner limbs 

 of the folds form the true amnion (Fig. 508, a.p.), the outer limbs 

 form the false amnion or chorion (Fig. 508, s.z.m.). 



At the point where the folds finally meet their fusion is in- 

 complete and the inner and outer limbs remain connected (sero- 

 amniotic connection). 



The cavity bounded by the true amnion contains a fluid which 

 bathes the outer surface of the embryo ; that between the amnion 

 and the chorion is Uned by mesoblast and is continuous with the 

 coelomic space between the yolk sac and the overlying somato- 

 pleure. As the split between the layers spreads round the yolk 

 sac, the outer layer it forms continues the chorion, which finally 

 surrounds the sac. Meanwhile the folding off of the embryo 

 has narrowed the connection between it and the rest of the 

 blastoderm, so that the amniotic cavity encloses the embryo 

 except in the region of this narrow umbilical stalk in the middle of 

 the belly. The amnion provides a bath for the embryo, which 

 is no more able to stand drying up than an amphibian larva is, 

 and, as a cushion, protects it against injury when the egg is 

 moved, and helps in thermal insulation. At a later stage (eleven 

 to fifteen days) a channel is formed through which egg-white 

 flows into the amniotic cavity, and then the whole contents pass 

 into the intestine and are there digested and absorbed. 



While the amnion is being formed, a sac known as the allantois 

 (Fig. 508, 4, al.) grows out from the hinder part of the gut of 

 the embryo, in the position in which the urinary bladder stands 

 in the adults of those vertebrate animals which possess it. This 

 sac, like the gut from which it grows out, is lined with endoderm 

 and covered with splanchnic mesoderm, and projects into the 

 extra-embryonic coelom. It grows down the umbilical stalk and 

 spreads out between the amnion and the chorion, fusing by its 



