Both the cytological and the genetic evidence shows that when two X chromosomes are present a female is produced, when one, a male.
"Sex-linked Inheritance in Drosophila"
Thomas Hunt Morgan Calvin B. Bridges
Two students working in Professor Wilson's laboratory have obtained definite evidence in favor of the cytological explanation of Mendel's principles, and have thus made an important step in the solution of one of the important fundamental mysteries of cell development in the very early life of organisms.
"Catholic Churchmen in Science"
James J. Walsh
In another way he made a contribution of a more positive character, for his elaborate speculations as to the genetic meaning of cytological appearances have led to a minute investigation of the visible phenomena occurring in those divisions by which germ-cells arise.
"Darwin and Modern Science"
A.C. Seward and Others