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 
peculiarly delicate covering, the first fetal membrane, or 
amnion, which commences at the navcl, is formed round 
the embryo; this membrane is filled with the amnion- 
water, and encloses the embryo or germ in the form 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 Ammnion- 
animals (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-nostriled and skull-less animals), may on 
the other hand be opposed to the others as anvnion-less 
animals (Anamnia). 
The formation of the fcetal 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 
