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 



FIG. 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. c. 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. 



