MhY 



1902 



No. 1 — CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY 

 OF THE MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD 

 COLLEGE, UNDER THE DIRECTION OF E. L. MARK, No. 130. 



Changes accompanying the Migration of the Eye and Observations 

 on the Tractus o'pticus and Tectum opticum in Pseudopleuro- 

 nectes america7ius. By Stephen E. Williams. 



TABLE OF CONTENTS. 



I. Introduction 1 



II. Material 2 



III. Methods 6 



IV. Migration of the eye and 



changes in the cartilagi- 

 nous skull G 



1. Summary of previous studies 



on the migration of the eye 6 



2. Description of stages ... 9 



3. Homologies of the anterior 



bones of the skull ... 11 



4. Changes in the cartilaginous 



skull 15 



a. Stage I. 15 



b. " II 16 



c. " III a 19 



d. " III ^ 22 



e. "IV 25 



PAGE 



f. Comparisonof Bothus with 

 Pseudopleuronectes ameri- 

 canus 28 



g. Discussion of Pfeffer's 

 work oO 



h. Resume 32 



V. The optic portion of the cen- 

 tral nervous system ... 33 



1. General condition in the adult 33 



2. The optic nerves .... 35 



3. The chiasma and tracts with 



related ganglia 37 



4. The tectum opticum . . 40 

 VI. Theoretical considerations . 47 



VII. Summary 49 



Bibliography 51 



Explanation of Plates 56 



I. Introduction. 



The strarige want of symmetry iii, tlic head region of flounders 

 has attracted much attention especially because in adults both eyes 

 occupy the same side of the head. The peculiarity is the more re- 

 markable because, for some time after hatching, the eyes and all otlier 

 parts of the head are as symmetrical as in any other fish, and conse- 

 quently this asymmetrical condition is brought about afresh in the 

 individuals of each generation, instead of once for all, as is the case 

 with most variations. 



Regarding the migration of the eye, with a single exception (Pfeffer, 

 '86, '94), only such phenomena liave been recorded as can be observed 

 from surface study or dissections. It has seemed desirable therefore to 



VOL. XL. — NO. 1 1 



