I Ichthyopsida. This section comprises the Fishes and the Amphibians, and is characterised by the presence at some period of life of gills or branchiae, the absence of an amnion, the absence or rudimentary condition of the allantois, and the possession of nucleated red blood-corpuscles.
II. Sauropsida. - This section comprises the Birds and the Reptiles, and is characterised by the constant absence of gills, the possession of an amnion and allantois, the articulation of the skull with the vertebral column by a single occipital condyle; the composition of each ramus of the lower jaw of several pieces, and the articulation of the lower jaw with the skull by the intervention of an "os quadratum;" and, lastly, the possession of nucleated red blood-corpuscles.
III. Mammalia. - This section includes the single class of the Mammals, and agrees with the preceding in never possessing gills, and in having an amnion and allantois. The Mammalia, however, differ from the Sauropsida in the fact that the skull articulates with the vertebral column by two occipital condyles; each ramus of the lower jaw is simple, composed of a single piece, and the lower jaw is united with the temporal (squamosal) element of the skull, and is not articulated to a quadrate bone. There are special glands - the mammary glands - for the nourishment of the young for a longer or shorter period after birth, and the red blood-corpuscles are non-nucleated.