HEREDITY 



171 



the germ are held on a very fine balance. A very slight impulse 

 the one way or the other determines the sex direction the em- 

 bryo shall take. Although much investigation and very much 

 speculation have been devoted to this problem, it is still ] un- 

 solved. We are not able, in the vertebrate animals, nor in fact 

 in animals generally, to determine the nature of the stimulus, or 

 of any of the various impulses, if more than one exists, which 



to.. 



"to,. 



FIG. 103. Limb skeletons of various animals, showing homologous bones: 9, Orni- 

 thorhynchus; 10, kangaroo; 11, Megatherium; 12, armadillo; 13, mole; 14, sea 

 lion; 15, gorilla; 16, man. 



leads the individual germ cell to develop as male or female. 

 It is also possible that each germ cell is really bisexual from the 

 beginning. One sex or the other becomes dominant and the 

 other recessive as the embryo develops. But in this event we 

 are still in doubt as to the nature of the determining factor or 



1 The latest studies of the problem are chiefly concerned with an attempt 

 to determine whether or not there exists a chromosome sex determinant, 

 and whether sex determination may not be brought under Mendel's law 

 of heredity (see later paragraphs in this chapter) in a modified form. 



