264 DR. TRAQUAIR ON THE ASYMMETRY OF THE PLEURONECTIDE. 
that nature, in order not to lose an eye, was necessitated to put it into the hollow of 
the right cheek under the single remaining orbit." 
Rosenthal (Ichthyotomische Tafeln, Berlin, 1812-1822), a little more rational in his 
ideas than Autenrieth, held the upper eye of the Flounder to be that of the left or now 
eyeless side, but accounted for its getting to the right side by supposing its being 
thrust through the head, and getting ** placed between the long processes of the frontal 
bones after the manner of Cyclopean malformations." The view which occurred to him 
is then in accordance with the second theory suggested in the beginning of this paper, 
but which a careful examination of the osteology of a series of different species of flat- 
fish will easily show to be untenable. 
It is, however, to Meckel* that we owe most of our previous knowledge of the sub- 
ject. He recognized correctly the homologies of the various eranial bones with those of 
the symmetrical fish, and was undoubtedly the first who saw clearly that, according to 
the first theory already mentioned, the two eyes of the flatfish are brought round to one 
side by a twisting process; but his notions as to the prolongation of the dorsal fin 
along the head are unsatisfactory, as we shall see afterwards. | 
Van Beneden, in 1853, published a paper À, the first in which, so far as I know, notice 
has been taken of the development of the Pleuronectidæ. In this paper he has de- 
scribed a young Turbot taken probably soon after its extrusion from the egg, and in 
which that stage of development does not seem yet to have been reached when the eyes 
become both placed on one side. “In this young fish the mouth is perfectly symme- 
trical; the eyes are still on the two sides of the head, but the left is about to pass over 
to the right side; the nostrils are still symmetrical. The rays of the dorsal fin only yet 
descend to the middle of the eranium ; afterwards they stretch on in front of the eyes; 
but it is necessary first that the twisting of the head should have taken place on the 
vertebral column." To these observations he adds the result of some made “on à 
Turbot of nearly adult size, in which the process of torsion is arrested when the eye has 
arrived at the middle line. The rays of the dorsal fin have not yet descended to more 
than in the embryo described ; the two sides are equally coloured." In remarking upon 
this paper, I may say that here, for the first time, do we find distinetly announced the 
fact and doctrine that the dorsal fin is not primarily advanced so far forwards on the 
head as we find it in the fully developed flatfish, but that it advances after the eye has 
turned round, and then it proceeds straight forwards, regardless of the deviation of the 
original mesial line of the head. Thus we are afforded a ready and rational explana- 
tion of the diffieulty which met us at first, namely as to how, if the middle line of the 
top of the head has been twisted over to one side, the dorsal fin, a mesial structure, has 
not followed that twisting. Van Beneden, however, is not the first to notice an ocea- 
sional condition of the adult flatfish resembling that which he has described in his 
2 ba Syst. der vergl, Anatomie, Theil ii.: Halle, 1824. Meckel's first observations on the subject appeared in a paper 
Ueber die niche Asymmetrie im thierischen Körper,” in his‘ Anatomisch-physiologische Untersuchungen, = 
822, a work which I have not had an opportunity of seeing. T See p. 287, note. 
t “Note sur la Symétrie des Poissons Pl : ge sich 
euro te i ” Sci N tur elles, 
XX. pp. 340-342, nectes dans le jeune âge,” Ann. des Sciences Nat: 
