EMBRYOLOGY 507 



the mesoblast into somatic and splanchnic layers has pro- 

 gressed some way outwards from the embryo over the yolk, 

 there arise upward folds' 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 



Fig. 3S6. — View of mammalian embryo, with its fcetal membranes. 

 — After Kennel. 



an:., Amnion proper; d., dwindled yolk sac; aL, allantois; al'., subzonal 

 membrane; z., z'., villi. Outside the subzonal membrane there is the 

 delicate ectodermic trophoblast (s.cfi.). 



form the true amnion, the outer limbs form the false 

 amnion. The cavity bounded by the true amnion contains 

 a fluid which bathes the outer surface of the embryo ; that 

 between the true and false amnions is lined by mesoblast 

 and continuous with the ccelom of the embryo. As the 

 split between the layers spreads round the yolk sac, the 

 outer layer it forms continues the false amnion, which 

 finally encloses the sac. Meanwhile the folding off of the 

 embryo has narrowed the stalk of the yolk sac, so that the 

 amniotic cavity encloses the embryo except in the region 



