196 



THF DEVELOPMENT OF THE CHICK 



at the same time the convex wall becomes thin, and the concave 

 thick. The entire tubule thus becomes S-shaped. Figs. 114 A, 

 B, C, D illustrate the corresponding processes in the duck, which 

 are similar in all essential respects to the chick. 



B 



• r 



C D 



Fig. 114. — From a transverse series through a duck embryo of 45 s, to 

 show the formation of the mesonephric tubules. (After Schreiner.) 

 Fig. 218 shows the position of the sections A, B, and C. 

 V. c. p., Posterior cardinal vein. W. D., Wolffian duct. 

 A. and B. represent tubules of the twenty-ninth segment. 



C. of the twenty-seventh segment. 



D. of the twenty-fourth segment. 



In the chick embryo of 35 somites the only differentiated 

 tubules are in front of the twentieth somite, a region of the 

 mesonephros that never develops far, and such tubules do not 

 appear ever to become functional. In the region of the subse- 

 quent functional mesonephros (twentieth to thirtieth somites) the 

 development has not progressed beyond the stage of the vesicles 

 showing the first indications of budding. 



