. The chordates. Chordata. Reproduction 279. Fig. 230 (Left). Diagrammatic transverse section of the body of a vertebrate embryo at an advanced stage. The muscle-forming myotome is beginning to extend into the ventral body-wall of the embryo, (c) Coelom; (g) genital ridge; (m) muscle derived from myotome; (mc) myocoele; (p) peritoneum; (pd) pronephric duct; (so) somatic layer (dermatome) of somite; (v) advancing ventral border of myotome. The finely dotted areas are occupied by mesenchyme. (Courtesy, Kingsley: "Com- parative Anatomy of Vertebrates," Philadelphia, The Blakiston Compan
. The chordates. Chordata. Reproduction 279. Fig. 230 (Left). Diagrammatic transverse section of the body of a vertebrate embryo at an advanced stage. The muscle-forming myotome is beginning to extend into the ventral body-wall of the embryo, (c) Coelom; (g) genital ridge; (m) muscle derived from myotome; (mc) myocoele; (p) peritoneum; (pd) pronephric duct; (so) somatic layer (dermatome) of somite; (v) advancing ventral border of myotome. The finely dotted areas are occupied by mesenchyme. (Courtesy, Kingsley: "Com- parative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) Fig. 231 (Right). Diagrammatic transverse section of the body of a vertebrate, (av) Aorta; (c) coelom; (e) ectoderm; (ep) epaxial (dorsal) muscle; (g) gonad; (ha) hemal rib; (hp) hypaxial (ventral) muscle; (i) intestine; (mes) mesentery; (n) kidney (nephridium); (o) omentum; (p) somatopleure (peritoneum); (r) dorsal rib; (sp) splanchnopleure; (V) centrum of vertebra and, above it, neural arch containing spinal cord. (Courtesy, Kingsley: "Comparative Anatomy of Vertebrates," Philadelphia, The Blakiston Company.) development of the kidney, the primary tubule may produce branches which become additional renal tubules. In vertebrates other than fishes and amphibians, the embryonic coelom does not extend into the mesomeric zone and the material of the zone does not become definitely segmented—, no distinct "-meres" are formed. The renal tubules develop as solid cords of mesoderm, the cords later hollowing out to become tubules. The development of renal tubules begins in the more anterior region of the mesomeric zone and progresses posteriorly. Certain dif- ferences in mode of development and eventual structure compel the distinction between an earlier and more anterior system of tubules, the pronephros (Fig. 229), and a later more posterior and more ex- tensive system, the mesonephros (Fig. 80). In fishes and amphibians, the mesonephros becomes the adul
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