WILLIAMS: MIGRATION OF EYE IN PSEUDOPLEUEONECTES. 7 



The first paper which really describes a method of transition of the 

 eye in flatfishes is that of Steenstrup ('63). According to Wyville 

 Thomson ('65), on whose abstract of Steenstrup's paper I have relied 

 (see also Steenstrup, '64), this author contends that the final posi- 

 tion of the eyes cannot be explained as simply the result of a torsion of 

 the front part of the head ; and there is, in his (S.'s) opinion, a pene- 

 tration of the tissues of the head by one of the eyes. This process 

 Steenstrup described carefully from alcoholic specimens of different sizes 

 of the young forms which he provisionally termed Plagusise. In this 

 species development resulted in a sinistral flounder, i. e., one in which 

 the left side during adult life is uppermost. The right eye was slightly 

 in advance of, as well as dorsal to, the left eye. The mouth became 

 oblique toward the blind side, and the posterior part of the face, where 

 the normal eye is located, seemed pressed " upward " toward the future 

 eye-side. The right eye no longer projected from its own side of the 

 head in a large orbit, but was deeply imbedded in the tissues, so that it 

 had only a small orbit-opening on the right side. Later, an opening was 

 made on the left side and for a time the eye had two orbits. The orig- 

 inal orbit soon closed, and as the eye reached the surface level on the 

 left side of the head the new orbit increased in size. This second orbit- 

 was described by Thomson as a bony one in the adult fish, being formed, 

 so Thomson contended, by the frontal and prefrontal of both sides. 



Schiodte ('68), working on other species, showed that the passage of 

 the eye around the 'head is a normal method of development. The 

 penetration of the eye through the tissues of the head is restricted to a 

 few fishes whose larval forms were once considered adults, and given the 

 name Plagusia. 



He observed a Pleuronectes platessa — a dextral flounder — 10 milli- 

 metres long, of which he says, " The right eye stands over the beginning 

 of the lower third of the maxillary bone. The left eye stands at the top 

 of the head, so much inclined to the right that from the left side only 

 slightly more than one-third of the pupil can be seen ; it stands in front 

 of the dorsal fin, so that the latter is just behind the end of the left and 

 [the] beginning of the middle thirds of the eye." In a 14 mm. speci- 

 men the pupil of the left eye had become invisible from the left side 

 and the dorsal fin touched the left margin of this eye, the foremost ray 

 being a little in advance of the extreme posterior margin of the eye. In 

 a 40 mm. fish the right eye had moved so that it stood over the lower 

 end of its maxillary bone and the left eye had followed it, so that they 

 were almost as close to each other as in the last stage, the left eye being 



