. Éléments de l'anatomie comparée. Anatomie, comparatif. J2G L'anatomie comparée. de leur connexion avec les parties de squelette interne. Non seulement ils forment un bouclier dorsal sur la surface dorsale, ventrale, mais l'une sur la face ventrale (plastron). Dans le bouclier dorsal nous pouvons faire une ligne médiane de l'os, qui sont fusionnés avec les épines des vertèbres, le projet et d'eux. Sur les côtés il y a des plaques plus grandes, qui sont soudés avec nervure-comme les processus, et en plus de ces il existe des plaques marginales autour du bord de l'écu. Ce sont désireux de Trionyx. Quatre pièces appariées un

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. Elements of comparative anatomy. Anatomy, Comparative. I2G COMPARATIVE ANATOMY. of their connection with the internal skeletal parts. They not only form a dorsal shield on the dorsal surface, but a ventral one on the ventral surface (plastron). In the dorsal shield we can make out a median row of bones, which are fused with the spines of the vertebras, and project from them. At the sides there are larger plates, which are fused with rib-like processes, and in addition to these there are special marginal plates around the edge of the shield. These are wanting in Trionyx. Four paired pieces and one unpaired piece can be made out, as a rule, in the plastron. All of these parts are variously developed in different families of the Chelonii. Although the dermal bones of the Eeptilia may probably be rightly regarded as derived from the bony carapace of Fishes, we must regard the ossifications which are found in the Edentata as independent arrangements, which have had their origin in fresh adaptive modifications. <^J Internal Skeleton. § 327. The internal skeleton is of greater morphological importance than the skeletal structures formed from the integument; it is connected, on the one hand, with arrangements which are found in the Iuver- tebrata, and on the other, and by a long- series of very varied arrangements, it can be followed out through all divisions of the Vertebrata. At first the internal skeleton has the form of a rod-like structure which tra- verses the whole length of the body, and is, when simplest, made up of indifferent cells, and surrounded by a cuticular structure which is formed from a secre- tion of these cells. This primitive organ of support is the chorda dorsalis or notochord ; we have already met with it in the Tunicata (cf. §303). The invest- ment formed by it is the chordal sheath (cs). The earliest rudiment of the noto- chord is placed just below the central nervous system; it has not always the same relations to the germ-layers, al- thoug

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