i; FLATFISHES (HFTFROSOMATA) 



change than the nitt-rorbital. As Regan has pointed out. however, embryological 

 work on the metamorphosis of Flatfishes, and more particularly on the development 

 of the cranium, provides no evidence in support of Traquair's view of the twisting of 

 the facial region of the skull. This work may now be brieliy considered. 



As is well known, when first hatched the larval Flatfish has one eye on each side 

 of the head, and swims vertically near the surface of the sea in the normal manner. 

 At an early stage one eye migrates round the upper surface of the head and comes 

 to lie more or less close to its fellow of the other side, and from this time onwards the 

 fish lies on the bottom with the ocular side uppermost. The larva has generally been 

 described as symmetrical when newly hatched, but, although this may be true of the 

 eyes, it is doubtful whether the developing skeleton, viscera, etc., are ever quite 

 symmetrical after the larva has left the egg, or, at any rate after the yolk-sac has 

 disappeared. The metamorphosis, and more especially the relation of the eyes to the 



chondrocranium. has been studied in some detail by Williams (1902) in Pseudopleiiro- 

 necles and Scophthahnus , by Mayhoft (1914) in Pleiironecles, and by Berrill (1925) in 

 Pleuronectes and Solea. At an early stage after hatching no cartilages are developed 

 above the eyes, but early in the metamorphosis the postorbital bar on each side grows 

 forward to meet the corresponding parethmoid process to form a complete supra-orbital 

 bar, which connects the ethmoid with the otic region of the chondrocranium. These 

 bars are, of course, the praecursors of the frontal bones, and are more or less completely 

 reabsorbed' before the bones commence to ossify. ."Mmost as soon as they are 

 completed, however, that of the blind side disappears, with the exception of the 

 postorbital portion and sometimes a small part at its anterior end. That is to say, 

 that part of the supraorbital bar which lies in the path of the migrating eye is ab.sorbed, 

 and the eye subsequently passes through the gap thus formed until it reaches the 

 ' The partial or complete absorption of the supra-orbital bars before ossification of the frontals 

 etc. takes place has been recorded in the ontogeny of several symmetrical Bony Fislies — sec Norman 

 (ir,:;f,A,p. 425). 



