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Show G4 ON OIJASBIFICA'riON. None are known to be provided with rnedian fins supported by fin-rays, and their limbs ar~ ~1ever. fringed wit~1 fin-rays. Furthertnore, in all Amphttbtta winch possess hn1bs, the skeleton of these limbs is divisible into parts which obviously correspond 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 arrangmnent to the humerus, rnclins and ulna, carpus, metacarpus, and phalang H; and, in the hind limb, to the femur, tibia and fibula, tarsus, 1netatarsus, and phalanges of the higher vertebrates. This is the case in no fi sh ; for, whether fishes possess parts corresponding with the lnunerus, radius and ulna, &c., or not, certain it is that the elements of their limb skeletons are very differently dispo ed frorn tho arrangement which obtains in Arnphibia and in higher vertebrates. In all Amphibia the skull articulates with the spinal column by two condyles, and the basi-occipital r n1ains unossifiecl. Furthermore, the cranial peduncle, or susponsoriu1n, to which the lower jaw is articulated, give. attachment to the hyoidean apparatus. These last are characters by which tho Amphibia are sharply distinguished from the higher vertebrate . There is a striking contrast between the lo. e 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, tho latter almost always taking on, directly or indirectly, a r spiratory function. rrhe amnion is a sac filled with fluid whi h envelopes and shelters the en1bryo, during its slow asFmTnption of the condition in which it is competent to breathe anclrecei ve food frmn without. The mode of its formation is shown in the accompanying figures of the early stages of develop1nent 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 "primiti vo groove," bounded at the sides by a slight elevation of the bla. todrrn1, indicating t]Jc THE AMPHIBIA. 65 position of the future longitudinal axis of the body of the chick. Soon, the lateral boundaries of this groove, in what will become Fig. 31. ~~·\\ . .:;,' ' ,.<~:"·' , .. \.\~1~1\\\111\ . ' .: ·'·i•.'/' D' 1' ''11 l\l~!!~'!~!iulll!!!!l!!il~~lk(b Fig. 31.-Development of the Chick. ~· i~~·st ru~i~en; of ~he embryo; a, its cephalic; b, its caudal end; c, primitive groove. · e em IY? ur~ er advanced; a, b, c, as before; d, the dorsal laminre develo ed in C L the ?:phalic r~giOn only, and near~y uniting in the middle line; e, the proto·veftebrre. · etten; ~~ be~Oie. The dors~l l~mmre have united throughout the greater part of the cephahc regwn, and are beg1nmng to unite in the anterior spinal region D. Embryo fu~ther advanced (second day), the dorsal laminre having unit~d throughout nearlhy 1 thelr wh?le J~ngth. The proto-vertebrre have increased in number and the , , omp a o-meseraiC vems, f, are visible. ' 1 he erdb~·yos a~·e drawn. of the same absolute length, but it will be understood that the o e1 emb1 yos are, m nature, longer than the younger. F |