STRUCTURE OF FOUR-DAY CHICKS 



145 



mesoderm, and are all composed of units which are tubular in 

 nature. In the different nephroi these tubules vary in struc- 

 tural detail but their functional significance is in all cases much 

 the same. They are concerned in collecting waste materials 

 from the capillary plexuses which are developed in connection 

 with them. In the accompanying diagrams conventionalized 



__ pronephric 

 tubules 



pronephric tubules J -*i- ; ^ 

 (degenerating) ~~| 



mesonephric I ^J/^Q 



tubules with \ 



nephrostomes 



mesonephric 

 tubules without 

 nephrostom 



mesonephric duct 



cloaca 



mesonephric -=r> 0) .'^ 



tubules ^"-^i /;' ;Ir> 



and duct ^i.^--! : ;r' ,-, 



degenerating /^ ^ M 



allantois 

 esonephric duct 

 nephric duct 



cloaca 



FIG. 51. Schematic diagrams to show the relations of pronephros, meso- 

 nephros, and metanephros at various stages of development. For explanation 

 see text. 



tubules have been drawn to represent the three nephric organs. 

 No pretense is made of representing either the exact shape or 

 the actual number of the tubules. 



In the first stage represented (Fig. 51, A) only the pronephros 

 has been established. It consists of a group of tubules empty- 

 ing into a common duct, called the pronephric duct. The pro- 

 10 



