CHAPTER XII. 



THE EXCKETORY SYSTE.M. 



The oxidation phenomena gronped under metabolism, and 

 involving both synthesis and destruction of cell materials, are 

 accompanied by the production of waste products. Such waste 

 products include carbon dioxide, mineral salts, water, and numerous 

 simple nitrogenous compounds. These are mainly remo\ed from 

 the organism in aqueous solutions. We have already observed 

 that in vertebrates carbon dioxide and some water are removed 

 from the blood through the respiratory system, and that the integu- 

 ment plays a part in the elimination of small amounts of waste. 

 However, among most animals a distinct excretory system has been 

 evolved to deal with nitrogenous wastes. 



In the chordates we find the presence of a segmental condition 

 in the excretory system. In the primitive forms there is a system 

 of segmental nephridial tubules with ciliated openings (nephro- 

 stomes) into the coelom and outlets direct to the exterior. With the 

 cephalochords such tubules have nephrostomes in the coelomic 

 cavity and are also associated with coils of blood vessels, so that 

 wastes are removed from both the coelomic fluid and the blood. 

 With the vertebrates there is an increasing efficiency in the develop- 

 ment of nephridial tubules for the remo^'al of wastes from the blood, 

 and they become organized into one of three types of excretory 

 organs called kidneys. These form in the embryo along the mid- 

 dorsal region, one on each side, at different levels. The three types 

 have a common source of origin in the mesoderm of the intermediate 

 cell mass, the nephrotome, which lies lateral to the mesodermal 

 segments and connects them with the somatic and splanchnic layers 

 of mesoderm enclosing the c(elom. Tubules formed from this 

 mesodermal region become grouped along the mid-dorsal region 

 to form each type of kidney. 



In the cyclostomes, young fish, and young amphibians, the 

 pronephros-, or head kidney, is functional; in adult fishes and amphib- 

 ians, the mesonepliro.s, or middle kidney, is functional; in rei)tiles, 

 birds, and nuinunals, the nictancpliro.s-, or ])()steri()r kidney, becomes 



