38 Congenital Nir/Jif-BUndness 



LEGENDS OF FIGURES. 



Fig. 1. Pedigree showing the inheritance of night-blindness in a Texas family. The 

 black symbols are the affected members. The Eoman numerals on the left refer to 

 generations. 



Fig. 2. Diagram to show the possible mechanism of the heredity of night-blindness. 

 The normal accessory chromosome is represented by X ; the abnormal accessory 

 chromosome by .t, which is assumed to carry the factor for night-blindness. In the 

 first generation x is in the male line ; in the second generation it always crosses over 

 to the female line and is dominated by A' ; in the third generation it should be equally 

 distributed between the male and female lines. A minus sign ( - ) indicates the 

 absence of the X or .r element. 



LITERATURE. 



1. Bateson, W. Menders Principles of Heredity. Cambridge, 1909. 



2. Davenport, C. B. Heredity in Relation to Eugenics. New York, 1911. 



3. GuvER, M. F. " Acces.sory Chromosonie.s in Man." Biol. Bull. Vol. xix. 1910. 



4. Nettleship, E. "A History of Congenital Stationary Night-blindness in 



Nine Consecutive Generations." 0}}hth. Sac. Trans, xxvii. 1907. 



5. — _. "Three new Pedigrees of Eye Disease." lldd. .xxvrii. 1908. 



