. Comparative anatomy. Anatomy, Comparative. 96 COMPARATIVE ANATOMY. mesomeres become transformed into kidney tubules. The process begins in the more anterior mesomeres and progresses posteriorly. Certain differences in mode of development and in eventual structure compel the distinction between an earlier and more anterior system of tubules, the pronephros, and a later and more posterior system, the mesonephros. In Anamnia the mesonephros becomes the adult kidney and the pronephros disappears except that in a few fishes it is the definitive and only kidney. In amniotes, following development
. Comparative anatomy. Anatomy, Comparative. 96 COMPARATIVE ANATOMY. mesomeres become transformed into kidney tubules. The process begins in the more anterior mesomeres and progresses posteriorly. Certain differences in mode of development and in eventual structure compel the distinction between an earlier and more anterior system of tubules, the pronephros, and a later and more posterior system, the mesonephros. In Anamnia the mesonephros becomes the adult kidney and the pronephros disappears except that in a few fishes it is the definitive and only kidney. In amniotes, following development of a pronephros and a mesonephros, the tubule-forming process continues backward, but with some modifi- cations and compKcations, to form a third kidney, the metanephros, which becomes the adult kidney. The tubular epididymis, associated with the testis of the adult amniote, is a part of the embryonic mesone- phros which otherwise disappears except for certain vestiges which are apparently of little functional importance. In Anamnia each mesomere at first forms a single renal tubule. In what is conceived to be the more primitive method of development of the tubules, the cavity is the original cavity of the mesomere and is there- fore a part of the embryonic coelom and continuous with the coelomic space of the hypomere. In amniotes, however, the mesomere material may be not only unsegmented but devoid of any original cavity. In such case the tubule shapes up as a solid cord, later hollowing out. Meanwhile, as the pronephric tubules form, the mesomere material on each side of the embryo gives rise to a longitudinal tube (Fig. 64) which extends backward from the pronephric region to the wall of the cloaca into which the cavity of the duct finally opens. The pronephric tubules of each side are joined to the corresponding pronephric duct (Fig. 70) thus putting the coelom into communication with the exterior by way of the cloaca. The coelomic openings or nephrostomes (Figs. 64B, n, a
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