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 rts Bearing on the Asymmetry 
of the Heterosomata (Flatfishes). 
By G. H. PARKER. 
TABLE OF CONTENTS. 
PAGE PAGE 
I tntroduction, . . .. . . 221 chiasmata of the Hetero- 
II. Positions of the nerves in the Somata:s. oot. mee, ie hg eee 
chiasmata of symmetrical IV. The asymmetry of the Hetero- 
teleosts "ce tse a ee BONNAGR, co “cla Jel) axles a peas 
III. Positions of the nerves in the Vv. ammary a.) SR) ees 
Biormmhy,, (s) 000) Siti Sn od ele 4 RD 
I. Introduction. 
THE optic chiasma in the great majority of teleosts is formed by a 
crossing of the optic nerves without an intermingling of their fibres ; 
hence these vertebrates are peculiar in that the two optic nerves can 
be readily dissected apart even at the chiasma. Since the organs con- 
nected by these nerves — the 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 left as applied 
to the optic nerves. Some authors, particularly the older ones, designate the nerve 
right or left depending upon the side of the 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 
