THE EXCRETORY ORGANS. 



109 



few epithelial tubes which take a somewhat twisting course, but may be said to 

 run, in general terms, transversely. Each tube begins with a ciliated funnel-shaped 

 opening (Fig. 63, /) not far from the median line of the embryo, and ends, after 

 a more or less contorted course, in a longitudinal duct, which, after receiving all 

 of the tubules, runs toward the posterior end of the embryo and opens into the 

 extremity of the entodermal or digestive canal. Opposite the funnels, and separate 

 from the pronephros proper, there is a so-called glomus (Fig. 63, gl), which is a 

 projection of not inconsiderable size from the mesentery. When fully developed 

 the glomus contains a rich network of blood-capillaries, so that it somewhat resem- 

 bles the glomerulus of the kidney. The circulation of the pronephros is sinusoidal. 

 The second of the excretory organs is termed the mesonephros, Wolffian body, 

 or fetal kidney. It is the only excretory organ in elasmobranchs. In adult am- 



nch 



Ec 



FIG. 63. FROG (RANA TEMPORARIA) TADPOLE OF 12 .o MM. CROSS-SECTION OF THE PRONEPHRIC REGION. 



nch, Notochord. m, Muscles. /, Pronephric funnel, v, Blood-vessel. EC, Ectoderm, t, Pronephric tubule. 



gl, Glomus. Lu, Lung. X 90 diams. (After M. Furbringer.) 



phibians it replaces the pronephros, which i purely a larval structure. It is pres- 

 ent in the embryos of all amniota, but undergoes a partial degeneration before 

 adult life, being itself replaced in adult amniota by the true kidney. The meso- 

 nephros resembles somewhat the pronephros, especially as found in the ichthyopsida. 

 It occupies a much larger region of the body than the pronephros. It has no 

 glomus associated With it, but each tubule contains a glomerulus very similar in 

 its general structure to the glomerulus of a true kidney. In the ichthyopsida each 

 tubule begins with a ciliated funnel, and, after making several coils, opens into the 

 pronephric duct. In the amniota the mesonephros, or, as it is more commonly 

 called in these animals, the Wolffian body, is essentially an embryonic structure. 

 Its tubules, however, do not have at any stage the ciliated funnels to be found in 

 amphibia and fishes, but they have glomeruli and they open into the pronephric 



