JAN 28 1905 



No. 5. — CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY 

 OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD 

 COLLEGE. E. L. MARK, DIRECTOR. No. 138. 



The Optic Chiasma in Teleosts and its Bearing on the Asymmetry 

 of the Heterosomata {Flatfishes). 



By G. H. Parker. 



TABLE OF CONTENTS. 



PAGE 



I. Introduction 221 



II. Positions of the nerves in the 

 chiasmata of symmetrical 



teleosts 222 



III. Positions of the nerves in the 



PAGE 



chiasmata of the Hetero- 

 somata 224 



IV. The asymmetry of the Hetero- 

 somata 233 



V. Summary 238 



Bibliography 239 



I. Introduction. 



The optic chiasma in the great majority of teleosts is formed by a 

 crossing of the optic nerves without an intermingUng of their fibres ; 

 hence these vertebrates are pecuHar in that the two optic nerves can 

 be readily dissected apart even at the chiasma. Since the organs con- 

 nected by these nerves — tlie eyes and the optic lobes — are, as a rule, 

 symmetrically disposed, it would seem a matter of indifference whether 

 an optic nerve in its course from the eye to the optic lobe should pass 

 in the chiasma dorsally or ventrally to" the other optic nerve. Appar- 

 ently very little attention has been given to this relation, for a search 

 through the papers on the cranial nerves of fishes has yielded only a 

 few scattered observations and general statements unsupported by much 

 evidence. Stannius ('49, p. 12) declared that for the most part the 

 nerve from the left side of the brain, that is, the right nerve,^ is dorsal at 



1 There has been some confusion in the use of the terms right and lejl as applied 

 to the optic nerves. Some authors, particularly tlie older ones, designate the nerve 

 right or left depending upon the side of tlie brain from which it arises ; others use 

 these terms in accordance with- the eye to which the nerve is attached. In this 

 paper the nerves are termed right or left depending upon their attachment to the 

 right or to the left eye. 



VOL. XL. — NO. 5 1 



