144 EARLY EMBRYOLOGY OF THE CHICK 



is tics of developing muscle cells. Their arrangement in bun- 

 dles which project toward the lumen fore-shadows the formation 

 of the muscle bands (trabeculae carneae) which ridge the inner 

 wall of the adult heart. The cells of the epicardial portion of 

 the epi-myocardium are becoming flattened to form the epi- 

 theUal and connective tissue covering of the heart (epicardium) . 

 Lying between the endocardium and the myocardium in the 

 region of the atrio- ventricular canal and of the opening of the 

 ventricle into the bulbo-conus, there are loosely aggregated 

 cells which are mesenchymal in characteristics. These cells 

 constitute what is called endocardial cushion tissue. They 

 later take part in the formation of the septa which divide the 

 heart into chambers and in the formation of the connective 

 tissue frame- work of the cardiac valves. 



VI. The Urinary System 



The General Relationships of Pronephros, Mesonephros 

 and Metanephros. — In the development of the urinary system 

 of birds and mammals there are formed in succession three dis- 

 tinct excretory organs, pronephros, mesonephros, and meta- 

 nephros. The pronephros is the most anterior of the three, 

 and the first to be formed. It is wholly vestigial, appearing 

 only as a slurred-over recapitulation of structural conditions 

 which exist in the adults of the most primitive of the vertebrate 

 stock. The mesonephros is homologous with the adult excre- 

 tory organs of fishes and amphibia. It makes its appearance 

 in the embryo somewhat later than the pronephros, and is 

 formed caudal to it. The mesonephros is the principal organ 

 of excretion during early embryonic Hfe, but it also disappears 

 in the adult except for parts of its duct system which become 

 associated with the reproductive organs. The metanephros 

 is the most caudally located of the excretory organs, and the 

 last to appear. It becomes functional toward the end of em- 

 bryonic life when the mesonephros is disappearing, and per- 

 sists permanently as the functional kidney of the adult. 



Figure 51 shows schematically some of the main steps in the 

 embryological history of the nephric organs, which it will be 

 helpful to have in mind before taking up in detail any of the 

 phases of their formation in the chick. The pronephros, meso- 

 nephros and metanephros are all derived from the intermediate 



