114 



COMPARATIVE PHYSIOLOGY. 



nected with the duct, finally join it. This organ is continuous 

 with the pronephros ; in fact, all three (pronephros, mesone- 

 phros, and metanephros) may be regarded as largely continua- 

 tions one of another. 



The metanephros, or kidney proper, arises from mesoblast at 

 the posterior part of the "Wolffian body. The ureter originates 



FiQ. 123. — Section of the intermediate cell-ma&B of fourth day (Foster and Balfour, 

 after Waldeyer). 1 x 160. m. mesentery; Z. somatopleure; a', portion of the 

 germinal epithelium from the duct of Miiller is formed hy involution; a, thick- 

 ened portion of the germinal epithelium^ in which the primitive ova, C and o. are 

 lying; M, modified mesoblast which will form the stroma of the ovary; WK, 

 Wolffian body; y, Wolffian duct. 



first from the hinder portion of the Wolffian duct. In the fowl 

 the kidney tubules bud out from the ureter as rounded eleva- 

 tions. The ureter loses its connection with the Wolffian duct 

 and opens independently into the cloaca. 



The following account will apply especially to the higher 

 vertebrates : 



The segmental (archinephric) duct is divided horizontally 

 into a dorsal or Wolffian (mesonephric) duct and a ventral or 

 Miillerian duct. The Wolffian duct, as we have seen, develops 

 into both ureter and kidney proper. 



To carry the subject somewhat further back, the epithelium 



