THE URINOGENITAL ORGANS OF VERTEBRATES. 155 



generative ducts of Lepidosteus to the urinary ducts is very 

 similar to that existing in other Ganoid fishes ; and this, 

 coupled with the fact that Lepidosteus possesses a pair of 

 abdominal pores on each side of the anus 1 , makes it most proba- 

 ble that its generative ducts are true Miillerian ducts. 



In the Amphibians the urinary system is again more primi- 

 tive than in the Selachians. 



The segmental duct of the kidneys is formed 2 by an elon- 

 gated fold arising from the outer wall of the body-cavity, in 

 the same position as in Selachians. This fold becomes con- 

 stricted into a canal, closed except at its anterior end, which 

 remains open to the body-cavity. This anterior end dilates, 

 and grows out into two horns, and at the same time its opening 

 into the body-cavity becomes partly constricted, and so divided 

 into three separate orifices, one for each horn and a central 

 one between the two. The horns become convoluted, blood 

 channels appearing between their convolutions, and a special 

 coil of vessels is formed arising from the aorta and projecting 

 into the body-cavity near the openings of the convolutions. 

 These formations together constitute the glandular portion 3 of 

 the original anterior segmental tube or segmental duct of the 

 kidneys. I have already pointed out the similarity which this 

 organ exhibits to the head-kidneys of Cyclostome fishes in its 

 mode of formation, especially with reference to the division of 

 the primitive opening. The lower end of the segmental duct 

 unites with a horn of the cloaca. 



After the formation of the gland just described the remainder 

 of the kidney is formed. 



1 This is mentioned by Muller (Ganoid fishes, Berlin Akad. 1844), Hyrtl (he. ?.), 

 and Gi.int.her (loc. cif.), and through the courtesy of Dr Gunther I have had an oppor- 

 tunity of confirming the fact of the presence of the abdominal pores on two specimens 

 of Lepidosteus in the British Museum. 



2 My account of the development of these parts in Amphibians is derived for the 

 most part from Gotte, Die Entwickelungsgeschichte der Unke. 



3 It is called Kopfniere (head-kidney), or Urniere (primitive kidney), by German 

 authors. Leydig correctly looks upon it as together with the permanent kidney con- 

 stituting the Urniere of Amphibians. The term Urniere is one which has arisen in 

 my opinion from a misconception ; but certainly the Kopfniere has no greater right lo 

 the appellation than the remainder of the kidney. 



