THE AMNION-SAC. 219 



In passing from the Amphibia to the next class of 

 Vertebrata, namely, Reptiles, we observe a very considerable 

 advance in the progress of organization. All the double- 

 nostriled animals (Amphirrhina) up to this time considered, 

 and more especially the two larger classes of Fish and 

 Batrachia, agree in a number of important characteristics, 

 which essentially distinguish them from the three remaining 

 classes of Vertebrata — Reptiles, Birds, and Mammals. 

 During the embryological development of these latter, a 

 IDeculiarly delicate covering, the first foetal membrane, or 

 amnion, which commences at the navel, is formed round 

 the embryo ; this membrane is filled with the amnion- 

 water, and encloses the embryo or germ in the fonn of a 

 bladder. On account of this very important and character- 

 istic formation, we may comprise the three most highly 

 developed classes of Vertebrata under the term Am^nion- 

 anivials (Amniota). The four classes of double-nostriled 

 animals which we have just considered, in which the 

 amnion is wanting (as is the case in all lower Vertebrate 

 animals, single-nosti-iled and skull-less animals), may on 

 the other hand be opposed to the others as amnion-less 

 anivials (Anamnia). 



The formation of the foetal membrane, or amnion, 

 which distinguishes reptiles, birds, and mammals from all 

 other Vertebrata, is evidently a very important process in 

 their ontogeny, and in the phylogeny which corresponds 

 with it. It coincides with a series of other processes, which 

 essentially determine the higher development of Amnionate 

 animals. The first of these important processes is the 

 total loss of gills, for which reason the Amniota, under the 

 name of Gill-less animals (Ebranchiata), were formerly 



