228 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



two heads were opposite. This is very significant, for it indicates that 

 the dimorphic condition of the chiasma is not hereditary. If heredity 

 is not a factor, what does determine the crossing of the nerves in the 

 chiasmata 1 In attempting to answer this question some other 

 possible factors were examined, but with negative results. 



TABLE VIII. 



Results obtained from Dissection of Monstrous Specimens 

 OF Salvelinus fontinalis. 



Results from Study of the Development of the 

 Optic Nerve. 



Histological investigation was made of the optic nerves in different 

 stages of development. For this purpose eggs were brought from 

 the hatchery at Wilkinsonville, Massachusetts, and kept in a trough 

 of running water. Eggs were taken each day and fixed in Vom Rath's 

 fluid, which proved to be the best fixative for this purpose. As is 

 well known, the great majority of nerve fibres in the optic nerve de- 

 velop from the retinal cells and grow backward to the brain. The 

 object of this investigation was not, however, to study the mode of 

 development, but rather to find out whether both nerves developed 

 at the same time. In nearly all cases the nerve fibres had reached the 

 brain by the tenth day. Whenever the nerve fibres could be traced, 

 they developed at the same time, for the fibres fi"om both eyes were 

 equally developed. The dimorphism is not due to an earlier develop- 

 ment of one of the optic nerves. 



