186 THE FROG. 



cavity by three ciliated mouths or nephrostomes ; the tubules 

 are surrounded by, or rather imbedded in, the large posterior 

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

 excretory matters are separated. The excretions are carried 

 away from the head kidneys by a pair of tubes, the segment al 

 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 appear, the adult kidneys, or Wolffian bodies, 

 begin to form in the hinder part of the body, as a series of tubes 

 which grow towards, and open into, the segmental ducts (Fig. 

 83, KM) ; these ducts carrying away the excretory matters 

 separated by both the larval and adult kidneys. 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 8G) 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 relation with the 

 segmental ducts, or actually from these ducts ; and the Wolffian 

 bodies become so closely related to the reproductive organs in 

 both sexes, that it is desirable to describe the development of 

 the two systems, urinary and reproductive, together. 



2. The Head Kidney and Segmental Duct. 



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

 of between 3^ and 4 mm. length, as a longitudinal ridge of the 

 somatopleuric mesoblast, immediately below the ventral borders 

 of the myotomes, MS. This ridge lies immediately beneath the 

 epiblast, 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 groove communicating 

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

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



