852 HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 



not part of the true kidney. From my own studies on young 

 Fishes I do not believe that the oldest larvae investigated by 

 Rosenberg were sufficiently advanced to settle the point in 

 question ; and, moreover, as Rosenberg had no reason for doubt- 

 ing that the so-called head-kidney of the adult was part of the 

 excretory organ, he does not appear to have studied the histo- 

 logical structure of the organ which he identified with the em- 

 bryonic pronephros in his oldest larva. 



The facts to which I have called attention in this paper 

 demonstrate that in the Sturgeon the larval pronephros un- 

 doubtedly undergoes atrophy before the adult stage is reached. 

 The same is true for Lepidosteus, and may probably be stated 

 for Ganoids generally. 



My observations on Teleostei are clearly not sufficiently ex- 

 tensive to prove that the larval pronephros never persists in this 

 group. They appear to me, however, to shew that in the normal 

 types of Teleostei the organ usually held to be the pronephros 

 is actually nothing of the kind. 



A different interpretation might no doubt be placed upon 

 my observations on Lophius piscatorius, but the position of the 

 kidney in this species appears to me to be far from affording a 

 conclusive proof that it is homologous with the anterior swelling 

 of the kidney of more normal Teleostei. 



When, moreover, we consider that Lophius, and the other 

 forms mentioned by Hyrtl as being provided with a head-kidney 

 only, are all of them peculiarly modified and specialized types 

 of Teleostei, it appears to me far more natural to hold that their 

 kidney is merely the ordinary Teleostean kidney, which, like 

 many of their other organs, has become shifted in position, than 

 to maintain that the ordinary excretory organ present in other 

 Teleostei has been lost, and that a larval organ has been retained, 

 which undergoes atrophy in less specialized Teleostei. 



As the question at present stands, it appears to me that the 

 probabilities are in favour of there being no functionally active 

 remains of the pronephros in adult Teleostei, and that in any 

 case the burden of proof rests with those who maintain that 

 such remnants are to be found. 



The general result of my investigations is thus to render it 

 probable that the pronephros, though found in the larva or em- 



