THE LATER DEVELOPMENT OF THE CHICK 355 



1. The Excretory System 



The intermediate cell masses, or nephrotomes, form the 

 rudiments of the excretory system which, as in all Amniota, is 

 complicated by the succession of three nephric systems, pro- 

 nephros, mesonephros, and metanephros, of which the first two 

 are purely embryonic, only the last giving rise to the excretory 

 system of the adult. The nephroi develop only through the 

 neck and trunk regions, for in the head and tail no lateral plate, 

 nephrotome, and somite are differentiated in the mesodermal 

 segment. 



A. THE PRONEPHROS AND THE PRONEPHRIC DUCT 

 (WOLFFIAN DUCT) 



The pronephros is wholly of vestigial character in the chick, 

 functionless even in the embryo. The pronephric duct, how- 

 ever, is retained as the duct of the embryonic kidney, and is 

 hence known as the mesonephric or Wolffian duct. On 

 account of its vestigial character the pronephros develops 

 variably, even in different regions in a single individual. It is 

 limited to the fifth to fifteenth or sixteenth somites, but 

 becomes typically developed only in the tenth to fifteenth. 



In the latter region a small bud of cells grows upward from 

 the middle of the postero-dorsal surface of each nephrotome. 

 These buds, appearing about the middle of the second day, are 

 the rudiments of the pronephric tubules, so-called although they 

 remain solid here. The buds or tubules elongate gradually, and 

 their terminal portions bend over posteriorly, each uniting with 

 the next posterior tubule, forming thus a continuous longitudi- 

 nal strand, which is the rudiment of the pronephric or Wolffian 

 duct. Toward the close of the second day the duct becomes 

 hollow anteriorly. It pushes backward rapidly, above the 

 nephrotomes, growing independently, until about the sixtieth 

 hour it reaches the cloaca, with which it fuses; its lumen is 

 completed throughout at the end of the third day. In front 

 of the tenth somite no duct is formed and the tubules are re- 



