DEVELOPMENT OF THE AMNION. 663 



here the epiblast is external. The true amnion is continuous with 

 the skin of the embryo, and when the foetus is mature, the connec- 

 tion may be traced by the umbilical cord, around which it forms 

 a sheath to be continuous with the skin at the umbilicus. This 

 membranous sac enlarges, and in mammalia eventually becomes 

 the large bag of liquid which contains the foetus. The amniotic 



Diagrammatic longitudinal section of a chick on the fourth day. (Allen 

 Thomson.) ep, Epiblast. Ay, Hypoblast. m, Somatopleure. v.m, Splanch- 

 nopleure. qf-pf, Folds of the amnion. pp, Pleuroperitoneal cavity, am, 

 Cavity of amnion. al, Allantois. a, Position of the future anus, h, Heart, 

 t, Intestine, vi, Vitelline duct, ys, Yolk, s, Foregut. m, Position of the 

 mouth, me, The mesentery. 



liquid is of low specific gravity, consisting mainly of water con- 

 taining traces of nitrogenous matter, and also phosphates and 

 chlorides. 



It contains albumin and some other nitrogenous constituents, 

 and a minute quantity of urea, which is thought to be derived 

 from the fcetal kidneys. 



This fluid preserves the child from the effects of any jolts or 

 jars caused by the movements of the mother, and similarly pro- 

 tects the uterus of the mother by acting as a buffer between the 

 foetus and the uterine wall. Before delivery it helps to dilate the 

 os uteri, so that when the amnion is ruptured the head of the 

 foetus occupies the opening which has been gradually made by 

 the fluid wedge. The outer part of the amniotic membrane, de- 



