DEVELOPMENT OF KIDNEYS AND FAT-BODIES IN THE FROG. 135 



further details. He describes the head kidney and its duct as arising 

 as a groove in the somatopleure, the lips of which close to form a 

 tube ; this tube opens in front into the ccelom by a longitudinal 

 slit-like mouth, which by fusion of its lips at two places gives rise to 

 the three nephrostomial apertures of the head kidney. Of these 

 three apertures, the two anterior ones lie close together, the first at a 

 more ventral level than the second ; the third aperture is some little 

 distance behind the other two. 



Owing to unequal rates of growth of the various parts, this 

 anterior portion of the tube, with the three nephrostomes, becomes 

 situated at a more dorsal level than the hinder part of the tube, 

 which now opens into it, not as at first behind the third nephrostome, 

 but on its ventral surface between the second and third nephro- 

 stomes. This hinder portion of the duct rapidly elongates and forms 

 an S-shaped loop lying ventral to the nephrostomes. In the later 

 stages the convolutions become more marked, and are further com- 

 plicated by lateral diverticula, of which there are three principal 

 ones, which intertwine amongst the convolutions of the main tube 

 and give off secondary branches, which, like the main diverticula, 

 end blindly. In this way the head kidney is formed as a com- 

 plexly twisted tubular gland, roughly spherical in shape, and 

 imbedded in the somatopleure at the anterior end of the body cavity. 

 The nephrostomial apertures become drawn out into tubes with the 

 growth of the gland, but retain their openings into the ccelom so long- 

 as the gland itself exists. The ducts soon acquire openings into 

 the cloaca at their posterior ends ; along the greater part of their 

 course they are flattened dorso-ventrally, and irregularly swollen at 

 intervals. Both the head kidney and its duct are, according to 

 Fiirbringer, distinctly and exclusively mesoblastic structures. 



Selcnka* gives an account of the structure and relations of the 

 head kidney and its duct in two young embryos of Hylodes 

 Martinicensis. He had no opportunity of working out the details of 

 development ; but his figures, reconstructed from a series of sections, 

 are of great excellence, and by far the most instructive that have 

 yet been published. The extreme irregularity in size of the tubules 

 of the head kidney, and the marked asymmetry between the organs 



* Selenba, " Dcr limbryonale Secretionsapparat des Kiemcnlos llylnik'S Martini- 

 ,'' Berlin, 1882. 



