KEPORT OF COMMISSIONERS OF INLAND FISHERIES. 51 



secondary adaptation to different external conditions and to different 

 methods of securing food. 



The process b}^ which the asymmetry of the eyes is reached in the 

 course of the development of the individual is an interesting one. 



Young flat-fishes, when first hatched out from the egg, are sym- 

 metrical, and swim upright. A little later, while the bones of the 

 head are still in a cartilaginous condition, the fish turns over on one 

 side, and in the course of a few days the eye of the under side has 

 moved clear around so as to take up its position on the upper side. 

 The details of this transformation have been a subject of a lively 

 controversy for the last half century. Steenstrup (1863) first 

 studied the migration of the eye; he described it as sinking in 

 through the tissues of the head and coming out on the other side. 

 Malm (1868) maintained, on the contrary, that in the species ob- 

 served by him the eye which belonged to what was to become the 

 under side of the fish, moved over the profile of the head to take its 

 position on the upper side. After some years of controversy it was 

 found by Alexander Agassiz, and others, that both these early 

 investigators were right, and that each had described the process as 

 it actually seemed to take place in the species with which he was 

 working. It has, however, now been established that the process 

 does not essentially differ in either case; according to this view% the 

 asymmetry of the head is not primarily due to the mere migration 

 of the eye from the under side of the head, but is the result of a 

 twisting of the whole ocular region of the head and involves an 

 extensive torsion of the bones of the orbital and preorbital region. 

 The process has been described as follo^^s: "the wdiole of the 

 cranium in the region of the orbit rotates on its longitudinal axis 

 until the two eyes instead of occupying a horizontal plane have 

 assumed a vertical one"* and one eye is dorsal to the other. At 

 least this seems to be true for species in which the development has 

 been completely worked out. If, at the time of metamorphosis, the 

 dorsal fin does not extend down over the frontal region of the head, 



* Cole and Johnstone, Pleuronectes, L. M. B. C. Memoirs. 1901, 9. 



