H. H. Newman 33 



his eyes he would need to hold a book. The right eye looks straight 

 ahead, but the left does not focus with the right. In looking closely at 

 objects he turns his head to one side and looks with the left eye. He 

 can hold his hand over the left eye and see well with the right. He is 

 being treated for this trouble by an oculist who claims that he will be 

 able to see at night when he has finished with him." 



26. Ediuard T. Floyd, dead. Lived long enough to exhibit sure 

 signs of night-blindness. No facts about associated defects. 



V, 27—30. Issue of IV, 15 and 16. 



27. Floyd Hurst, dead, normal. 



28. Mildred Hurst, aet. 6, normal. 



29. Jack Hurst, aet. 4, night-blind, but without other defects. 



30. Marvin Hurst, aet. 8 months (will be nearly 2 years when this 

 paper is published. All the ages used in paper apply to the Autumn 

 of 1911). He now shows unmistakable signs of night-blindness, but 

 no other defects. 



V, 31—34. Issue of IV, 19 and 20. 



31. J. L. Montgomery, aet. 24, normal. 



32. R. D. Montgomery, aet. 21, night-blind and myopic (wears 

 concave glasses). 



33. H. L. Mo7itgomery, aet. 18, night-blind, myopic. Mr C. D. 

 Uzzell (III, 5), his great-uncle, says of him : " His eyes are in terrible 

 condition. He can hardly see in the daytime. If he starts to go 

 through a door without his glasses he is as apt to miss it as to go 

 through it." 



34. Evolyn Montgomery, aet. 8, normal. 



V, 35. Issue of IV, 23 and 24. Name and age unknown. Night- 

 blind. 



Discussion. 



The probable mechanism of the inheritance of night-blindness. 



It has been shown by Guyer ('10) that there is a bivalent X 

 chromosome in the spermatogonia of man and that two classes of 

 spermatozoa are formed in equal numbers, half with an X element and 

 half without^ In the light of the investigations of E. B. Wilson and 

 others it seems practically certain that the sex of man is in some way 



1 Some of the details of Guyer's results have been disputed, but need not be discussed 

 here. 



Jouip. of Gen. iii ^ 



