THE AMPHIBIA. 65 



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, it is certain that the elements of 

 their limb skeletons are very differently disposed from the 

 arrangement which obtains in Amphibia and in higher verte- 

 brates. 



In all Amphibia the skull articulates with the spinal column 

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



This is a character by which the Amphibia are sharply dis- 

 tinguished 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 

 embryonic state, a well-developed amnion and allantois, the 

 latter almost always taking on, directly or indirectly, a respi- 

 ratory 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. 33, 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 

 position of the future longitudinal axis of the body of the chick. 

 Soon, the lateral boundaries of this groove, in what will become 

 the anterior region of the body, grow up into plates the dorsal 

 laminae (Fig. 33, B); and these dorsal lamina?, at length 

 uniting, inclose the future cerebro-spiual cavity (Fig. 33, C, D). 

 The blastoderm, beyond the region at which the dorsal laminae 

 are developed, grows downwards to form the ventral laminae, and 

 where the margins of these pass into tho general blastoderm, 



F 



