102 



THE EARLY DEVELOPMENT OF MAMMALS. 



funnels, and separate from the pronephros proper, there is a so-called glomus 

 (Fig. 47, 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 resembles 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 foetal kidney. It is absent in many fishes, but it is well developed in 

 elasmobranchs. In adult amphibians it replaces the pronephros, which is purely 

 a larval structure. It is present 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 mesonephros resembles somewhat the pronephros, especially 



Fig. 47. — Frog (Rana temporaria) Tadpole of 12.0 mm. Cross-section of the Pronephric Region. 



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



,^l. Glomus. Lu, Lung. X 9° diams. — {After M. Fiirbringer.) 



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 beg;ins with a ciliated funnel, and, after 

 making several coils, opens into the pronephric duct. The circulation of the 

 organ is sinusoidal. 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 

 duct, which, on account of its relations to the organs, is in this type more com- 

 monly spoken of as the Wolffian duct. The circulation of the organ is sinusoidal. 



