202 LIFE: ITS NATURE AND ORIGIN 



injections of female hormones, roosters become "broody" and will 

 hatch out and care for chicks; while under the influence of male 

 hormones., hens become combative and will "tread" other hens. 

 Injections of male hormones make non-singing female canaries 

 assume male courtship behavior and sing like males. Chick em- 

 bryos of both sexes develop combs and spurs, but those of the 

 female soon degenerate. If the left (functional) ovary is removed 

 from a very young female fowl, the bird assumes male character- 

 istics, its right ovary becoming functionally a testicle; and it is 

 reported that a normal fertile hen has been changed to a "normal" 

 fertile rooster. 42 



Although Prof. C. E. McClung 43 discovered the sex chromo- 

 somes about half a century ago, it is only recently that their 

 operative mechanism has been made evident. The expression 

 "genie balance" applies to sex characteristics and indicates that 

 many genes play a role. 



The action of sex-controlling genes, presumably through sex 

 substances formed by their activities, may be glimpsed from the 

 following resume of the development of the sexual system in 

 amphibians, based mainly on Needham's description. 



What are to become the germ cells arise from the dorsal wall of 

 the gut endoderm. They become embedded in the wall of the 

 splanchic mesoderm as it moves in to form a median dorsal mesentary 

 plate. This plate splits lengthwise into two bundles which move 

 outwards. Next, chains of cells from the blastema of the mesonephros 

 migrate into the gonad and form a medulla or central core. The 

 endodermal germ-cells are thus connected with two sorts of mesoderm: 

 a cortex having female potentialities, and a medulla having male 

 potentialities; and no other mesoderm has like competence. It had 

 been shown by M. Laulanie 44 that in the case of a genetic female, the 

 cortex develops at the expense of the medulla and induces the for- 

 mation of eggs; whereas in a genetic male, the medulla develops at the 

 expense of the cortex, and induces the formation of sperms. Though 

 egg cells and spermocytes both have the unique power of reduction 

 division which halves the chromosome number,* the two haploid 

 sperms survive, one with an x chromosome, the other with a y chromo- 

 some; but only one of the haploid egg cells (with an x chromosome) 

 survives, the other being aborted and cast out as a "polar body." This 

 recalls, though at much different structural levels, the freemartin 

 effect above mentioned, and also the instinct which leads a queen 

 bee to sting to death all other queens in her domain. 



* The subsequent longitudinal splitting of the chromosomes in "equatorial divi- 

 sion" does not affect the haploid number. 



