148 THE URINOGENITAL ORGANS OF VERTEBRATES. 



In Osseous fishes the segmental duct of the kidneys developes, 

 as the observations of Rosenberg 1 (" Teleostierniere," Inaug. 

 Disser. Dorpat, 1867) and Oellacher (Zeitschrift fiir Wiss. Zool. 

 1873) clearly prove, by an involution from the body-cavity. 

 This involution grows backwards in the form of a duct and 

 opens into the cloaca. The upper end of this duct (the most 

 anterior segmental tube) becomes convoluted, and forms a 

 glandular body, which has no representative in the urinary 

 apparatus of Selachians, but whose importance, as indicating the 

 origin of the segmental duct of the kidneys, I have already 

 insisted upon. 



The rest of the kidney becomes developed at a later period, 

 probably in the same way as in Selachians ; but this, as far as I 

 know, has not been made out. 



The segmental duct of the kidneys forms the duct for this 

 new gland, as in embryo Selachians (Fig. 2), but, unlike what 

 happens in Selachians, undergoes no further changes, with the 

 exception of a varying amount of retrogressive metamorphosis 

 of its anterior end. The kidneys of Osseous fish usually extend 

 from just behind the head to opposite the anus, or even further 

 back than this. They consist for the most part of a broader 

 anterior portion, an abdominal portion reaching from this to the 

 anus, and, as in those cases in which the kidneys extend further 

 back than the anus, of a caudal portion. 



The two ducts (segmental ducts of the kidneys) lie, as a rule, 

 in the lower part of the kidneys on their outer borders, and open 

 almost invariably into a urinary bladder. In some cases they 

 unite before opening into the bladder, but generally have inde- 

 pendent openings. 



This bladder, which is simply a dilatation of the united 

 lower ends of the primitive kidney-ducts, and has no further 

 importance, is almost invariably present, but in many cases lies 

 unsymmetrically either to the right or the left. It opens to the 

 exterior by a very minute opening in the genito-urinary papilla, 

 immediately behind the genital pore. There are, however, a 

 few cases in which the generative and urinary organs have a 



1 I am unfortunately only acquainted with Dr Rosenberg's paper from an ab- 

 stract. 



