336 M. Agassiz on Fossil Fishes. 



and the rank which the families ought to occupy in the ich- 

 thyological scale. 



These comparative studies have led our author to the dis- 

 covery of a capital fact, which had not previously been an- 

 nounced, namely, that a remarkable parallelism exists between 

 the development of the individual and the development of the 

 entire class in the order of time. In the earlier periods of 

 embryonic life, no vertebral column exists. This organ is 

 represented, in embryos, by a gelatinous cord, which is called 

 the dorsal cord. It is around this organ, which continues for 

 a longer or shorter period in all fishes, that the vertebrae are 

 formed in the shape of osseous rings. These rings insensibly 

 enlarge, and always encroach more and more on the dorsal 

 cord, which at last altogether disappears in the majority of 

 fishes. There are certain types, however, the sturgeon, for 

 example, where it remains during the whole life ; accordingly, 

 this fish has no vertebrae, and the apophyses rest immediately 

 on the dorsal cord. Now, M. Agassiz makes us acquainted 

 with the fact, that this is likewise the case with the fishes of 

 the ancient epoch. All of them possess distinct spiny apo- 

 physes, often very strong and completely ossified, but they ex- 

 hibit no traces of distinct vertebrae ; whence the author con- 

 cludes that they were deprived of these organs, and that the 

 dorsal cord continued in them during the whole period of life, 

 as in the sturgeon. 



We may make a single remark with regard to the relative 

 superiority of the living types. Here, also, embryology every- 

 where reveals to us a wonderful parallelism. There is no fish, 

 however imperfect it may be, whose organization does not cor- 

 respond to one or other of the phases of life in the most per- 

 fect types. Let us take the lamprey for an example, or that 

 still more imperfect fish known by the name of Amphioxus or 

 BrancMostoma^ and which was arranged by Linnaeus among 

 the Vermes, so widely does it differ from ordinary fishes. Of 

 these two types, the first has only the cartilaginous base of 

 the cranium ; the second is completely destitute of it, and the 

 dorsal cord extends as far as the extremity of the muzzle. 

 The first has only a single fin, more or less separated ; in the 

 second, this fin uniformly surrounds the entire body. Finally, 



