THE LATER DEVELOPMENT OF THE CHICK 359 



from the cells of the nephrotomes of the last two or three 

 segments of the body (31-33). In this region 

 the nephrotomal structure is not clearly differen- 

 tiated, and it is referred to simply as the meta- 

 nephrogenous tissue. As the metanephric diver- 

 ticulum grows out it is accompanied on its inner 

 face, throughout all its branching, by cells of this 

 tissue. During the seventh or eighth day typ- 

 ical nephric vesicles appear, like those of the 

 mesonephros, and acquire openings into each 

 branch of the collecting duct; Malpighian bodies 

 develop in the usual manner. During the next 

 three or four days the entire metanephros is 

 established, and the mesoclermal cells surround- 

 ing the tubules and ducts form the stroma and 

 capsule of the kidney. During the fifth and 

 sixth days the terminal portions of the Wolffian 

 ducts and the metanephric diverticula become 

 absorbed into the wall of the cloaca so that the 

 ureters acquire openings separate from those of 

 the Woffian ducts; the latter are hence, after the 

 degeneration of the mesonephros, not at all ex- 

 cretory in function. No nephrostomes appear 

 in connection with the metanephros. 



From the preceding description it will be seen 

 that the inner or medullary part of the defini- 

 tive kidney is derived from the branched out- 

 growth from the Wolffian duct, while the outer 

 cortical layer of secretory tubules and Malpig- 

 hian bodies is derived from the metanephro- 

 genous tissue (nephrotomes) of the last two 

 or three body somites, and is therefore homo- 

 logous with the glandular part of the meso- 

 nephros. 



2. The Reproductive System 



Before describing the development of this system it seems 

 necessary to recall, in a few words, the composition of the adult 



FIG. 141 . 

 Diagram of the 

 arrangement of 

 the nephric ele- 

 m e n t s in the 

 ehick. After 

 Felix. Proneph- 

 ric duct, and me- 

 tanephric ducts 

 (ureters) in 

 black; mesoneph- 

 ric tubules cross 

 hatched; meta- 

 nephric tubules 

 in dotted out- 

 lines. 



