HEAD-KIDNEY IN ADULT TELEOSTEANS AND GANOIDS. 15 



larval organ, which either disappears or ceases to be functionally 

 active in the adult state. 



Eosenberg, to whom the earliest satisfactory investigations on 

 the development of the Teleostean pronephros are due, stated 

 that he had traced in the Pike [Esox hicins) the larval organ into 

 the adult part of the kidney, called by Hyrtl the pronephros ; 

 and subsequent investigators have usually assumed that the so- 

 called head-kidney of adult Teleosteans and Ganoids is the 

 persisting larval pronephros. 



We have already seen that Rosenberg was entirely mis- 

 taken on this point, in that the so-called head-kidney of the 

 adult is not part of the true kidney. From my own studies 

 on young Fishes I do not believe that the oldest larvse investi- 

 gated by Eosenberg were sufficiently advanced to settle the point 

 in question; and, moreover, as Eosenberg had no reason for 

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

 of the excretory organ, he does not appear to have studied the 

 histological structure of the organ which he identified with the 

 embryonic 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 exten- 

 sive to prove that the larval pronephros never persists in this 

 group. They appear to me, however, to show 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 Lophms 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 Hyitl as being provided with a head-kidney 

 only, are all of them peculiarly modified and specialised 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 specialised 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 



