The organs which we have now enumerated in this general survey, and of which we have noted the characteristic disposition, are those parts of the organism that are found in all vertebrates without exception in the same relation to each other, however much they may be modified. We have chiefly had in view the transverse section of the body (Figs. 101, 102), because in this we see most clearly the distinctive arrangement of them. But to complete our picture we must also consider the segmentation or metamera-formation of them, which has yet been hardly noticed, and which is seen best in the longitudinal section. In man and all the more advanced vertebrates the body is made up of a series or chain of similar members, which succeed each other in the long axis of the body—the segments or metamera of the organism. In man these homogeneous parts number thirty-three in the trunk, but they run to several hundred in many of the vertebrates (such as serpents or eels). As this internal articulation or metamerism is mainly found in the vertebral column and the surrounding muscles, the sections or metamera were formerly called pro-vertebræ. As a fact, the articulation is by no means chiefly determined and caused by the skeleton, but by the muscular system and the segmental arrangement of the kidneys and gonads. However, the composition from these pro-vertebræ or internal metamera is usually, and rightly, put forward as a prominent character of the vertebrate, and the manifold division or differentiation of them is of great importance in the various groups of the vertebrates. But as far as our present task—the derivation of the simple body of the primitive vertebrate from the chordula—is concerned, the articulate parts or metamera are of secondary interest, and we need not go into them just now. Fig.103, A, B. C, D. Instances of redundant mammary glands and nipples (hypermastism). Fig. 103 _A, B, C, D._—Instances of redundant mammary glands and nipples (_hypermastism_). _A_ a pair of small redundant breasts (with two nipples on the left) above the large normal ones; from a 45-year-old Berlin woman, who had had children 17 times (twins twice). (From _Hansemann._) _B_ the highest number: ten nipples (all giving milk), three pairs above, one pair below, the large normal breasts; from a 22-year-old servant at Warschau. (From _Neugebaur._) _C_ three pairs of nipples: two pairs on the normal glands and one pair above; from a 19-year-old Japanese girl. _D_ four pairs of nipples: one pair above the normal and two pairs of small accessory nipples underneath; from a 22-year-old Bavarian soldier. (From _Wiedersheim._)