18& THE FEOG. 



ca^y b y three ciliated mouths or neplirosta mes : t he tubule s 

 are surrounded by . or rather imbedded in, th e lar^e poster ior 

 cardinal veins , and it is from the blood in these yeins that the 

 excretory matters are separated. The excretions are carried 

 away from the head kidneys by a pair of tubes, the segmental 

 ducts, KA, which run along the dorsal wall of the body to its 

 hinder end, where they open into the cloaca, or hinder part of 

 the alimentary canal. 



The head kidneys and their ducts are well developed in the 

 tadpole at the time of hatching ; they subsequently increase 

 considerably in size, and are the sole excretory organs of the 

 tadpole during the earlier stages of its existence. About^the 

 time ^the hind legs appea r, the nrlnl^, jfidnryTj rr Wnlffinif bodies- 

 be gin to form in the hinder part of the bndv ^ as a series of tub es 

 w hich STOW towards, a nd open into, the segmental du cts (Fig. 

 83, km) ; thegg dnots carrvinfr awa y the excretor y mat ters 

 separated by both the larval and adult kidn eys. The Wolffian 

 bodies rapidly increase in size and in complexity, especially at 

 their posterior ends, and by the time of the metamorphosis 

 (Figs. 85 and 86) have attained considerable dimensions. The 

 head kidneys at the same time undergo degenerative changes, 

 and gradually disappear, while the Wolffian bodies, growing still 

 larger, become the kidneys of the frog. 



The genital ducts are formed in close relatio n with J he 

 s ej^mental ducts, or actually from these ctucts ; and the Wolffian 

 b odies beco me so closely related to the reproductiveorgans in 

 b pth sexe s, t hat it is desirable to describe the develo pment of 

 the.^wosystems, urinary and reproductive, together. 



2. The Head yii^nfiy ami Sfifj-mfi ntal Duct . 



The segmental duct (Fig. 70, Kb) first appears, in embryos 

 of between 3^ and 4 mm. length, as a lo ngitudinal ridge of th e 

 s pmatopleuric mesoblas t, immediately below the ventral borders 

 of the myotomes, MS. T his ridge lies immediately beneat h the 

 egiblagt, but is, in the frog, distinct from this at all stages of its 

 development. It is difficult to determine whether or not it is 

 solid at its first appearance, but it very soon becomes grooved 

 along its inner Surface (Fig. 70) ; the gi'oove communicating 

 with the body cavity, and the eMire ridge having, in transverse 

 section, the appearance of a fold of the mesoblast, with a 



