On the Development of the Pleuronecticlce. 221 



men described by him has one eye on each side, but above 

 the left eye is a little slit where the other should appear. 



In addition, M. Steenstrup remarks that the osteology of 

 the head of the adult flat fish confirms his view of the process 

 of ocular transposition in the embryo. Like Rosenthal he 

 compares the head of a flat fish to that of a cyclopean mal- 

 formation ; and affirms that the position in which we find 

 the upper eye is not homologous with that occupied by the 

 lower, nor with the orbit of any other fish or vertebrate ani- 

 mal in general. Here I will in the meanwhile only say that 

 my own dissections have led to a very opposite conclusion, 

 namely, that the upper eye of a flat fish is homologous in 

 position with the lower one — that the bar of bone be- 

 tween the eyes is the only representative of the frontal arch 

 in the symmetrical osseous fish, but altered in position, be- 

 ing pushed over to one side, and that the other bar of bone 

 bounding the orbit and upper eye on the inner side is an 

 entirely additional formation, developed from the frontal and 

 prefrontal of the eyeless side, and not found in the plan of 

 the ordinary fish head. 



Although M. Steenstrnp's observations are very remark- 

 able, and not to be put aside simply because they do not tally 

 with our preconceived notions, yet on the other hand I do 

 not think that we can at present accept them implicitly as 

 representing the normal process of development among all the 

 Pleuronectidse, especially seeing that they are in such direct 

 opposition to the teachings of the structure of the adult 

 pleuronect, both normal and malformed, and to the embryo- 

 logical observations, so far as they have gone, of other 

 authors. M. Steenstrup's specimens may possibly be malfor- 

 mations of a nature essentially similar to those already re- 

 ferred to, but in which the dorsal fin may have actually 

 fcidged over the upper eye. Or there may be some groups 

 of Pleuronec tithe, in which the upper eye in the normal 

 course of development may become bridged over by the ad- 

 vancing dorsal fin, a slit being left through which afterwards 

 the upper eye passes to its position on the binocular side of 

 the fish. This is, however, mere theory ; meanwhile it is 

 greatly to be desired that additional information be collected 



