SIGNIFICANCE OF THE AMN1ON. 



389 



it produces the tufted chorion, the hollow, branched tufts of 

 which grow into the depressions in the mucous membrane 

 of the maternal uterus. 



FIG. 142. Transverse 

 section through au em- 

 bryonic Chick in the 

 shoulder region (at the 

 fifth day of incubation). 

 The sectionpasses midway 

 between the rudiments of 

 the anterior limbs (or 

 wings, E). The amnion- 

 folds have grown com- 

 pletely together over the 

 back of the embryo. 

 (After Eemak.) (Com- 

 pare, as regards other 

 points, with Figs. 139, 140, 

 and 141, and Plate V. 

 Fig. 14.) 



In human Phylogeny the amnion is particularly in- 

 teresting, because it is a peculiar characteristic of the three 

 higher classes of Vertebrates. Mammals, Birds, and Reptiles 

 alone possess it, and therefore these three classes are 

 grouped together under the name of Amnion Animals, or 

 Amniota. All Amnion Animals, including Man, are de- 

 scended from a common parent-form. All the lower Verte- 

 brates, on the contrary, entirely want this amniotic formation. 



Of the three bladder-like appendages of the embryo just 

 mentioned, the amnion has no blood-vessels at any period 

 of its existence. On the contrary, the two other bladders, 

 the yelk-sac and the allantois, are provided with large blood- 

 vessels, which accomplish the nutrition of the embryonic 



