27.2 OBSERVATIONS ON THE DEVELOPMENT OF 
‘Embryologie des Salmones’ (1842), which forms a part of M. 
Agassiz’s ‘Poissons d’Eau douce.” At p. 256 of this excellent 
monograph, Vogt says— 
“The curvature of the extremity of the chorda dorsalis, which 
begins to be apparent in the Coregonus a short time before it is 
hatched, and attains its greatest amount about six weeks later, is 
another peculiarity of the embryos which deserves to be taken into 
consideration, because it subsequently disappears, and exists in adult 
fishes only in some genera of existing Ganoids and Placoids. These 
relations have not escaped the notice of observers, and M. Von Bar 
particularly expresses himself as follows.” 
Voet here gives the preceding citation from Von Bar, and then 
continues : 
“This peculiarity, together with many other features characteristic 
of embryos, has naturally led me to examine into the relations which 
exist between these modifications and the characters which dis- 
tinguish the fossil fishes of different geological epochs. 
It is a fact well known to all anatomists, that the vertebral elisnin 
of cartilaginous fishes does not terminate in the same way as that 
of osseous fishes; in the former the bodies of the vertebrae become 
successively smaller from before backwards, and incline upwards 
more or less towards the end of the tail, so that the part of the 
vertebral column which carries the rays of the caudal fin forms a very 
openangle with the longitudinal axis of the trunk. A very peculiar 
form of the caudal fin results from this disposition : instead of being 
symmetrically bifurcated, it is simply bilobed, in such a manner that 
the superior lobe, situated, like the inferior, under the prolongation 
of the vertebral column, extends further back than the latter, which 
is produced only by an elongation of the anterior rays of this same 
inferior side of the vertebra. It results from this, that the caudal 
fin of the Plagiostomes has, properly speaking, no rays? inserted in 
the upper face of its vertebre. 
“In osseous fishes, on the other hand, the vertebral column 
terminates behind in a great expansion, whose superior and inferior 
apophyses are strongly dilated, so as to form a large vertical plate, 
whose posterior edge is symmetrically truncated, so as to present 
an equal surface of attachment for the caudal fin-rays above and 
below the prolongation of the vertebral column. This caudal piece 
may be regarded as resulting from many vertebre soldered together, 
or else as a simple dilated vertebra carrying many vertical apophyses. 
The chorda dorsalis is continued in its interior, and is also a little 
1 This statement, however, is incorrect, as Miiller had long before shown. (T. H. H.). 
