. Chordate anatomy. Chordata; Anatomy, Comparative. REPRODUCTION 69. The epimere undergoes three kinds of diflferentiation. Its heavier inner wall is mainly converted into striated body-muscle, not only the dorsal but the ventral muscle. The myotome material grows ventral- wards, pushing its way between the ectoderm and the somatic meso- derm, until it reaches the mid- ventral plane (compare Figs. 60 and 61). The medial region of the epimere gives rise to loosely aggre- gated cellular masses (mesen- ch5mae) surrounding the notochord and neural tube (Figs. 58,60). This material produces such su
. Chordate anatomy. Chordata; Anatomy, Comparative. REPRODUCTION 69. The epimere undergoes three kinds of diflferentiation. Its heavier inner wall is mainly converted into striated body-muscle, not only the dorsal but the ventral muscle. The myotome material grows ventral- wards, pushing its way between the ectoderm and the somatic meso- derm, until it reaches the mid- ventral plane (compare Figs. 60 and 61). The medial region of the epimere gives rise to loosely aggre- gated cellular masses (mesen- ch5mae) surrounding the notochord and neural tube (Figs. 58,60). This material produces such supporting structures—connective tissue, carti- lage and bone—as may later be developed around these two axial organs. The thin outer wall of the epimere breaks up to form loose cellular masses, mesenchyme, which give rise to the dermis, the deeper fibrous and vascular layer of the skin. The terms myotome, sclerotome and dermatome are applied respec- tively to the muscle-forming, skeleton-forming and dermis-forming regions of the epimere. (Fig. 58) The mesomeres give rise to the tubular structures of the kidneys. The process begins in the more anterior mesomeres and progresses pos- teriorly. Certain differences in mode of development and in eventual structure compel the distinction between an earlier and more anterior system of tubules, the pronephros (Figs. 59,62), and a later more posterior and more extensive system, the mesonephros. In anamnia the meso- nephros 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 modifications, to form a third kidney, the metanephros, which becomes the adult kidney. The tubular epididjnnis, associated with the testis of the adult amniote, is a part of the embryonic mesonephros which otherwise disappears except for certain vestiges which are ap
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