RUDIMENTARY ORGANS OF ANIMALS 135 



yellow wings give the appearance of a leaf. The 

 neuters of Termites have no wings. 



6. Vertebrates. Among Vertebrates the genito- 

 urinary system contains a great number of rudi- 

 mentary organs. In order to fully understand the 

 nature of these organs, it will be necessary to 

 glance through the ontogenetic and phylogenetic 

 development of the system. 



The first stage in the formation of the kidney- 

 system is the pronephros (fig. 5 7, A). This primitive 

 organ consists of intricate canals (a, a, a), opening 

 into the body cavity at the point where the glomeruli 

 are formed on the sub-intestinal vein. All these 

 canals originally had apertures to the exterior. 

 Later on, however, these uriniferous tubules became 

 connected with one single excretory canal (c.) opening 

 into the cloaca (ce.). The primitive genital gland was 

 situated close to the pronephros. In process of time 

 the mesonephros replaced the pronephros (fig. 5*7,B); 

 in origin it was quite distinct from the pronephros, 

 its appearance being that of a secretory urinary 

 gland (G.) and its secretory canal (c.) (segmental 

 duct), was the same as that of the pronephros. 

 The urinary system thus formed, became and still 

 continues to be, closely connected with the genital 

 gland, the discharging canals of which passed 

 through the mesonephric kidney in order to find 

 a passage to the exterior through the segmental 

 duct. During the mesonephric stage, another canal 

 was formed which started from the cloaca and 



