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 



5 J 

 ^ 





pronephric 

 tubule 



\\lyr^'>jf\ mesonephric 

 tubules with 

 nephrostomes 



functional 

 mesonephros 



, - mesonephric 

 tubules 

 without 

 nephrostomes 



mesonephric duct 

 metanephric 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, ^) 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. 



