OF ARTS AND SCIENCES. 117 



IV. 



ON THE YOUNG STAGES OF SOME OSSEOUS FISHES. 

 By Alexander Agassiz. 



Presented Oct. 10th, 1877. 



I. Development of the Tail. 



The structure of the tail of bony fishes has been described by Agas- 

 siz, Vogt, Owen, Stannius, Heckel, Huxley, Kolliker, and Lotz. The 

 homocercal tails of bony fishes of the present day were contrasted by 

 Agassiz and Vogt, and subsequently by Heckel, with those of the 

 Ganoids, and of other fishes appearing before the Jurassic period, hav- 

 ing so-called heterocercal tails ; and they attempted to show that the 

 tails of all homocercal fishes pass during their development, through a 

 heterocercal stage. Heckel, Huxley, Kolliker, and Lotz have plainly 

 shown, that, while the external appearance of the tail of bony fishes is 

 homocercal, their real structure is only a modified heterocercal one : 

 so that, as far as we now know, the tail of all fishes is built upon the 

 modifications of the same type ; the caudal fin not differing (as I shall 

 show here), in its mode of development from the primitive embryonic 

 fin, from that of the dorsal or anal fins. The theory of Agassiz, that 

 the heterocercal tail of the young of bony fishes passes gradually into a 

 homocercal one, and that the tail of the young of the bony fishes repre- 

 sents an embryonic stage which is permanent in Ganoids, is apparently 

 overthrown by the well-established fact of the heterocercality of the 

 tail of adult bony fishes modified externally so as to assume a homocer- 

 cal form. 



In the following notes, I shall describe the gradual change of the 

 embryonic tail of several species of bony fishes, and call attention to 

 the presence of an embryonic caudal lobe, which has thus far, appar- 

 ently, escaped the notice of ichthyologists, and which shows remark- 

 ably well the identity of growth between the tails of Ganoids and of 

 bony fishes. 



As early as 1856, the late Professor Agassiz noticed in Lepidosteus 

 a peculiar fleshy filament (the extension of the vertebral column) above 



