136 ANIMAL BIOLOGY. [Parti. 



are developed the membranous and osseous labyrinths by pro- 

 cesses of differentiation. 



The Urinary Organs and Generative Ducts. The first trace 

 of the excretory system in the embryo is the segmented duct, which 

 is stated to take its origin from the epiblast, but soon appears as 

 a tube in the peritoneal epithelium of the somatic layer, near the 

 dorsal border of the body-cavity. This tube opens posteriorly 

 into the cloaca, and anteriorly into the peritoneal cavity by 

 apertures far forward, just behind the branchial region. Just 

 behind the openings the duct becomes coiled upon itself, and 

 sends out a number of blind diverticula. The glandular body 

 thus constituted is called the head-kidney or pronephros. Oppo- 

 site the openings of the duct, at the root of the mesentery, a 

 highly vascular ridge is formed, which constitutes a glomerulus. 

 The portion of the body-cavity which contains this glomerulus 

 and the openings of the segmental duct, is temporarily shut off 

 from the rest of the body-cavity. Thus a provisional and 

 temporary excretory organ is constituted. In the frog it sub- 

 sequently atrophies ; but in the cod-fish it probably persists 

 throughout life. 



Much further back in the body-cavity a number of coiled 

 tubes are formed. They originate in the mesoblast as solid, 

 cords, which are subsequently hollowed out into tubes, one end 

 of which opens into the segmental duct, while near the other 

 each is invaginated by a little tuft of vascular tissue, thus giving 

 rise to a glomerulus and its capsule (see p. 81). In some forms, 

 but not in the frog, there is one of these for each portion of the 

 body corresponding to a mesoblastic somite or segment ; they 

 are called segmental tubes. Secondary and tertiary segmental 

 tubes are subsequently added; they become much convoluted 

 and come into such close proximity that their primitive dis- 

 tinctness is completely lost. The large glandular body thus 

 constituted is called the Wolffian Body or Mesonephros, and 

 persists as the permanent kidney of the frog. 



In the meanwhile important changes go on in the segmental 

 duct of the frog. A cord of cells separates from its ventral side, 



