196 THE 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. 
Fic. 114. — From a transverse series through a duck embryo of 45s, 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. 
