04 ON CLASSIFICATION. 



Xone are known to be provided with median fins supported 

 by fin-rays, and their limbs are never fringed with fin-rays. 



Furthermore, in all Am/phibia which possess limbs, the skele- 

 ton of these limbs is divisible into parts which obviously corre- 

 spond with those found in the higher vertebrates. That is to 

 say, in the fore limbs there are cartilages, or bones, answering in 

 their essential characters and arrangement to the humerus, radius 

 and ulna, carpus, metacarpus, and phalanges ; and, in the hind 

 limb, to the femur, tibia and fibula, tarsus, metatarsus, and pha- 

 langes of the higher vertebrates. This is the case in no fish ; 

 for, whether fishes possess parts corresponding with the humerus, 

 radius and ulna, &c, or not, certain it is that the elements of 

 their limb skeletons are very differently disposed from the ar- 

 rangement which obtains in Amphibia and in higher vertebrates. 



In all Amphibia the skull articulates with the spinal column 

 by two condyles, and the basi-occipital remains unossified. 

 Furthermore, the cranial peduncle, or suspensorium, to which 

 the lower jaw is articulated, gives attachment to the hyoidean 

 apparatus. 



These last are characters by which the Amphibia are sharply 

 distinguished from the higher vertebrates. 



There is a striking contrast between the close affinity of the 

 fish and the amphibian and the wide separation of the Amphibia 

 from the succeeding classes, all of which possess, in the embry- 

 onic state, a well-developed amnion and allantois, the latter almost 

 always taking on, directly or indirectly, a respiratory function. 



The amnion is a sac filled with fluid which envelopes and 

 shelters the embryo, during its slow assumption of the condition 

 in which it is competent to breathe and receive food from with- 

 out. The mode of its formation is shown in the accompanying 

 figures of the early stages of development of the common fowl. 

 Fig. 31, A, represents the first step in the differentiation of the 

 embryo from the central portion of the blastoderm — that thin 

 membranous cellular expansion which lies on the surface of the 

 yelk where we see the cicatricula, or " tread." A well-defined, 

 though shallow, straight groove, the " primitive groove," bounded 

 at the sides by a slight elevation of the blastoderm, indicating the 



