24 
BULLETIN 905, U. S. DEPARTMENT OF AGRICULTURE. 
the twentieth century that the means by which sex is at least usually 
determined in the higher animals has been discovered. This mech- 
anism, however, seems to be one that is beyond human interference. 
It has been noted that a certain definite number of chromosomes 
can be seen under the microscope in the cells of each kind of animal. 
A qualification of this statement, connected with the determina- 
/-eme&le 
Ma,l& 
(#) 
( ••••*•• ) ( M»i ) 
( |m|m| j r |itii| J 
^Y^*^""/ 
<^ Spermi 
/m»i\ 
( !•«••! \ 
I ••••••• J 
I |i«|m|J 
Fema>le> 
Mate 
Fig. 6. — Diagram illustrating the method of sex determination in an animal with 14 chromosomes in the 
cells of the females, 13 in the males. All eggs have 7 chromosomes but only half the sperms have 7 chromo- 
somes, the other half having 6. The former are female-determining sperms, the latter male determiners. 
Hereditary factors carried by the sex-determining chromosomes pass with it from father to daughter, 
never from father to son in such cases as that illustrated above. 
tion of sex, was suggested hi 1902 by Prof. C. E. McClung, on the 
basis of a study of the cells of grasshoppers. This suggestion has 
since been confirmed in principle by a large number of scientists 
working with widely different kinds of animals, ranging from worms 
to mammals. In most cases that have been studied carefully a dif- 
ference has been found between the sexes, either in the number of 
the chromosomes or in the size of one pair. 
