298 CHORDATE ANATOMY 



GONADIC SAC FLATWORM. PORE 



I GONADIC SAC 

 GERM CELLS 



NEMERTEAN. PORE 



COELOMODUCT 



ANNELID. COELOM 



I VERTEBRATE EMBRYO. I 



PRONEPHRIC TUBULE PRONEPHRIC DUCT 



RENAL TUBULE 



' GERM CELL PRIMITIVE VERTEBRATE. MYOTOMES 



EFFERENT DUCTULE WOLFFIAN DUCT 



Fig. 268.— Diagrams illustrating the hypothetical evolution of the urogenital system 

 of vertebrates, beginning with the gonadic sacs of flatworms. The diagrams assume 

 that the gonadic sacs of flatworms have become the coelomic pouches of chordate 

 embryos. The latter in turn unite to form the extended coelom of vertebrates. Coe- 

 lomoducts appear first in annelids and become in vertebrates the renal tubules. Verte- 

 brates "invent" a longitudinal (primitive) excretory duct. During phylogenesis the 

 region of proliferation of germ-cells becomes greatly restricted. 



