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MANUAL OF PHYSIOLOGY. 



nection may be traced by the umbilical cord, around which it 

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

 This membranous sack enlarges, and in mammalia eventually 

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

 amniotic liquid is of low specific gravity, consisting mainly of 

 water containing traces of nitrogenous matter, and also phos- 

 phates and chlorides. 



FIG. 254. 

 



Diagrammatic sections of an embryo, showing the destiny of the yelk sack, ys vt. 

 Vitelline membrane, pp. Pleuro peritoneal cavity, ac. Cavity of the amnion. a. Am- 

 nion. a'. Alimentary canal, ys. Yelk sack. 



It contains albumin and some other nitrogenous constituents, 

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

 from the foetal 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 

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

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



