HEREDITY 



171 



the germ are held on a very fine bahxnce. A very shf^lit impulse 

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

 bryo shall take. Although much investigation and very nuich 

 speculation have been devoted to tliis 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 stinuilus, or 

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



m 



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 possi])le that each germ cell is really bisexual from the 

 beginning. One sex .or the other liecomes dominant and the 

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

 are still in doubt as to the nature of the determining factor oi- 



^ The latest studios of (ho prohlom aro rliiofly ronooriiod with an attompt 

 to dotormine whethor or not thoro (^xists a chroniosonio sex dof«'nuinaii( . 

 and whether sex determination may not bo hrou^ht under Mendel's law 

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



