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 — i.e., 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 adult kidney and the pronephros dis- 

 appears except that in a few fishes it is the definitive and only kidney. 

 In all other vertebrates, following development of a pronephros and a 

 mesonephros, the tubule-forming process continues backward, but with 



