. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 134 COMPARATIVE ANATOMY phylogenetic development of the fore-foot of the Horse, showing how it has been gradually derived from a tetra- or pentadaotyle form; and it has recently been ascertained that all these stages are passed through in the course of ontogeny. In this case the third digit becomes greatly enlarged relatively (perissodactyle form), and eventually is the only one remaining, while in cloven-footed Ungulates the third and fourth digits are both functional and equally strongly de- veloped (artiodactyle for

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. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 134 COMPARATIVE ANATOMY phylogenetic development of the fore-foot of the Horse, showing how it has been gradually derived from a tetra- or pentadaotyle form; and it has recently been ascertained that all these stages are passed through in the course of ontogeny. In this case the third digit becomes greatly enlarged relatively (perissodactyle form), and eventually is the only one remaining, while in cloven-footed Ungulates the third and fourth digits are both functional and equally strongly de- veloped (artiodactyle for
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Image ID: RDFX7P
. Elements of the comparative anatomy of vertebrates. Anatomy, Comparative. 134 COMPARATIVE ANATOMY phylogenetic development of the fore-foot of the Horse, showing how it has been gradually derived from a tetra- or pentadaotyle form; and it has recently been ascertained that all these stages are passed through in the course of ontogeny. In this case the third digit becomes greatly enlarged relatively (perissodactyle form), and eventually is the only one remaining, while in cloven-footed Ungulates the third and fourth digits are both functional and equally strongly de- veloped (artiodactyle form) and may be united together to form a "can- non-bone," the others becoming gradually reduced. A similar re- duction takes place in the hind- foot, and is here as a rule more rapid. Ungulates diverged into Artio- dactyles and Perissodactyles as far back as the Eocene period, but a large series of Tertiary forms shows that they must all have been derived from a common pentadactyle ances- tral form. Some of the many other adaptive modifications of the limbs in Mam- mals must also be briefly mentioned. In Bats, the phalanges are greatly elongated to support the wing- membrane ; the hallux as weU as the pollex may be opposable amongst the Primates; the fore-limbs are modified for digging in certain Mammals {e.g. Mole) ; and in the Cetacea (see p. 133) and Sirenia the digits are not free, and serve as supports for the fin-like paddles. Nails are present on the digits of Sirenia, but have disappeared in the Cetacea, though they can still he recognised in the embryo of toothed Whales. Hind-limbs are absent. Fig. 114.—Skeleton of the Left Fore-Limb or A, Pig; B, Htomos- CHus; 0, Tbagulus ; Z>, Roebuck ; E, Sheep ; F, Camel. (From Bell, after Garrod.) 1 VIVji II yrym a. Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and appearance of these illustrations may not perfectly resemble the

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