CONTRIBUTIONS FROM THE ZOOLOGICAL LABORATORY OF THE 

 MUSEUM OF COMPARATIVE ZOOLOGY AT HARVARD COLLEGE. 

 E. L. MARK, DIRECTOR. — No. 184. 



THE OPTIC CHIASMA OF TELEOSTS : A STUDY OF 



INHERITANCE. 



By a. p. Lakrabee. 



Presented by W. E. Castle. Received June 28, 190G. 



Introduction. 



In the majority of teleosts, as has been pointed out by several inves- 

 tigators, the condition of the optic chiasma is markedly different from 

 that found in other vertebrates. In these teleosts the nerve fibres of 

 the two optic nerves do not interlace at the chiasma, but each nerve 

 remains distinct, so that the two can easily be dissected apart at that 

 point. Conflicting statements regarding the condition of the chiasma 

 are found, some writers stating that the nerve running to the right 

 eye is dorsal, others making the opposite statement. In recent years 

 Parker ( : 03 ) has shown that one condition is about as common as the 

 other. He made a dissection of one hundred specimens, each often dif- 

 ferent species of symmetrical teleosts. Of these, 514 had the right 

 nerve dorsal and -18() the left nerve dorsal. In each case the right or 

 left nerve signifies the nerve running to the right or left eye respec- 

 tively. He showed, too, that this dimorphism is not correlated with 

 sex. To quote from his paper giving his conclusions on this point : 

 "In Fundulus, of the 51 specimens with the left nerves dorsal, 29 were 

 females and 22 males ; and of the 49 with the right nerves dorsal, 29 

 were females and 20 males. Of the 43 specimens of Tautogolabrus 

 with the left nerves dorsal, 26 were females and 17 males ; and of the 

 57 with the right nerves dorsal, 26 were females and 31 were males. 

 These figures show clearly that there is no close correspondence be- 

 tween the crossing of the optic nerves and sex." 



The question whether this dimorphism is due to racial differences 

 was left undecided. The writer will presently answer this question in 

 the negative, and at the same time discuss the relation of the dimor- 



