MULTICELLULAR ANIMALS 



.87 



the lowest of the vertebrates, the cyclostomes or lampreys. In the 

 fishes it appears in the embryo but is replaced by the second devel- 

 opment, the MESONEPHRos (Fig. 130). This consists of a considerable 

 number of collecting ducts, each with a special excretory organ, the 

 GLOMERULUS. The collecting ducts open into a mesonephric duct, 

 which is in reality the reorganized pronephric duct; it delivers the 

 wastes into the common receptacle, the cloaca. In vertebrates higher 



MESONEPHRIC 

 TUBULES 



DEGENERATING 

 PRONEPHRIC 

 TUBULES 



MESONEPHRIC 

 DUCT 



CLOACA 



Fig. 130. — Diagram of the vertebrate mesonephros. (After Patten: Embryology of 

 the Chick., published by P. Blakiston's Son and Co.) 



than the fishes, except mammals, both pronephros and mesonephros 

 appear during development, but the pronephros disappears, except 

 for its duct, and the mesonephros becomes the functional kidney. In 

 mammals, both pro- and mesonephros appear. The pronephros dis- 

 appears; the mesonephros either largely disappears as in females, or 

 becomes involved as an important structure of the male reproduc- 

 tive system. The definitive kidney or metanephros arises and be- 

 comes the functional excretory organ. The cloaca, which in the 



