334 ELEMENTS OF BIOLOGY 



Sex Determination. From what has been said concerning the 

 X chromosomes and their diflFerence in number in males and fe- 

 males, it would seem to be obvious that the factor which determines 

 sex is the presence or absence of one of the X chromosomes. Accord- 

 ing to this point of view, sex itself is a Mendelian character, the 

 female being homozygous and the male heterozygous. But the 

 problem is not so simple. It has been found that the sex chromo- 

 somes do not function as expected if certain allelomorphic genes 

 are absent from some of the autosomes. Moreover, there is a long 

 chain of developmental events, both structural and physico-chem- 

 ical, between the elemental condition when the fertilized egg re- 

 ceives into its nucleus a sperm nucleus, for instance containing an X 

 chromosome, and the completion to the adult female body. Nothing 

 whatever is known as to how this extra X chromosome so controls 

 the physico-chemical events that development results in femaleness, 

 nor why in its absence the chain of developmental processes results 

 in a male. Furthermore, there is substantial evidence that hormones, 

 derived from the interstitial cells (Figs. 136, 137) of the developing 

 gonads, influence sex development. By proper administration of such 

 hormones, either by injection or by grafting into the animal func- 

 tional glands of the opposite sex, it has been found possible to divert 

 a potential female toward maleness and, more rarely, to divert a 

 potential male toward femaleness. Complete sex reversal has been 

 accomplished experimentally in poultry and in Amphibia. A natural 

 demonstration of partial sex reversal in cattle is shown in the case of 

 the FREE MARTIN. A female calf born co-twin with a male is known 

 as a free martin and is almost always sterile. It has been found that 

 its sterility is due to a more or less complete reversion during de- 

 velopment of its reproductive apparatus toward the male type. Even 

 the ovaries exhibit certain structural characters that resemble testes. 

 An examination of the relation between these twins in cattle 

 showed that during the developmental period the embryonic cir- 

 culations are fused and continuous and that the bloods intermingle. 



