THE URINOGENITAL SYSTEM 379 
taneously: primary tubules are formed in each somite from the 
most ventral portion of the nephrogenous tissue; then secondary 
tubules later from an intermediate portion, and tertiary tubules 
later yet from the dorsal portion. 
Fig. 217 represents a transverse section through the middle 
7S. 
Ze Vm. 
Fic. 217. — Transverse section through the middle of the 
Wolffian body of a chick embryo of 96 hours. 
Ao., Aorta. Coel., Coelome. Col. T., Collecting tubule. 
Glom., Glomerulus. germ. Ep., Germinal epithelium. M_’s’t., 
Mesentery. n.t., Nephrogenous tissue. T. 1, 2,3, Primary, 
secondary, and tertiary mesonephric tubules. V. ce. p., Pos- 
terior cardinal vein. W. D., Wolffian duct. 
of the Wolffian body at the stage of ninety-six hours, showing a 
primary, secondary, and tertiary tubule. The primary tubule 
is typically differentiated; the secondary has formed the secreting 
tubule and the rudiment of the renal corpuscle, but the tubule 
does not yet open into the Wolffian duct, though it is connected 
with it; the tertiary tubule is still in the vesicular stage. Some 
undifferentiated nephrogenous tissue remains above the rudi- 
ment of the tertiary tubule, which makes it possible that quar- 
ternary tubules may be formed later. 
Referring still to the same figure, it will be noted that the 
Wolffian duct itself has formed a considerable evagination dorso- 
medially (collecting tubule), with which both secondary and 
tertiary tubules are associated as well as the undifferentiated 
nephrogenous tissue. Similar evaginations are formed along 
the entire length of the functional portion of the mesonephros. 
