Bmbryotogy 291 



While the amnion is lined with ectoderm and has its outer layer 

 formed of mesoderm, the chorion reverses this, having its ectoderm on 

 the outside and its mesoderm on the inside. The function of the chorion 

 is best explained along with that of the fourth embryonic membrane, 

 the allantois. 



UMBILICAL CORD 

 AMNIOTIC CAVITY 

 CHORION 

 VILLUS 

 EMBRYO 



Fig. 88. — Diagrams of sagittal sections to show embryonic membranes. A 

 to D, Chick. A, Amniotic folds forming; B, fold united; C, later stage with mem- 

 branes formed; D, amniotic folds forming in mammal; E, extra-embryonic struc- 

 tures of human embryo. (Redrawn with permission from Shumway: Introduction 

 to Vertebrate Embryology, 1942, John Wiley & Sons, Inc.) 



The mammalian embryo has both of these membranes well de- 

 veloped. Their development, however, is somewhat modified from that 

 of the reptiles and birds. In some the foldings are eliminated by the 

 appearance of splits which separate the future amniotic cavity from the 

 germinal disc. 



The Allantois. — The allantois is a saclike structure of splanchno- 

 pleure which grows out from the hind gut of the embryo. In birds and 

 reptiles it pushes out and comes to line the entire shell in the same way 

 that the chorion does. The outer wall of the allantoic sac fuses with the 

 overlying chorion to result in a common membrane lining the shell. 

 Blood vessels going through the mesodermal layers of these two mem- 

 branes serve for gaseous exchange; thus the allantois is the "lung" of 

 the embryo. 



