4. 



CORRELATION OF ENDOCRINE GLANDS WITH SEX DIFFERENTIATION 887 



animals, whereas in the development of many other animal species, only one of 

 the two sex possibilities becomes functional? 



b. Chromosomal, Sex-determining Mechanisms 



A considerable body of information has been obtained which demonstrates 

 a fundamental relationship between certain chromosomes and sex determina- 

 tion. The general topography of chromosomal sex-determining mechanisms 

 has been established for a large number of species. A pair of homologous 

 chromosomes, the so-called sex chromosomes, apparently have become spe- 

 cialized in carrying the genie substances directly concerned with sex determina- 

 tion. In many species, the members of this pair of sex-determining chromo- 

 somes appear to be identical throughout the extent of the chromosomes in 

 one of the sexes. In the other sex, on the other hand, the two sex-determining 

 chromosomes are not identical. When two identical chromosomes are present 

 in a particular sex, that sex is referred to as the homogametic sex, for the 

 reason that all of the gametes derived from this condition will possess identical 

 sex chromosomes. However, that sex which possesses the two dissimilar 

 chromosomes is called the heterogametic sex for it produces unlike gametes, 

 Often the heterogametic condition is represented by one chromosome only, 

 the other chromosome being absent. If under the above circumstances the 

 normally appearing chromosome is called X, and the deleted, diminutive or 

 strangely appearing chromosome is called Y, while the chromosome which 

 is absent be designated as O, we arrive at the following formula: 



XX rr the homogametic sex and either XY or XO = the heterogametic sex. 

 In many (probably in most) animal species the male is the heterogametic sex 

 (fig. 368A-C). 



In some animal groups, however, such as the butterflies, the moths, possibly 

 the reptiles, the birds, some fishes, and probably urodele amphibia, the female 

 is the heterogametic sex, and the male is homogametic. In these particular 

 groups, many authors prefer to use the designation ZZ for the homogametic 

 sex (i.e., the male) and ZO or ZW for the female or heterogametic sex. The 

 sex-determining mechanism in these groups, according to this arrangement, 

 will be ZZ:ZW or ZZ:ZO (fig. 368D). 



In endeavoring to explain the action of these chromosomal mechanisms, 

 one of the underlying assumptions is that the genie composition of the chromo- 

 somes actively determines the sex. For example, in cases where the female 

 sex is homogametic it is assumed that the X-chromosome contains genes which 

 are female determining; when two (or more) X's are present, the female 

 sex is determined automatically. When, however, one X-chromosome is pres- 

 ent, the determining mechanism works toward male determination. In those 

 species where the female sex is the heterogametic sex it may be assumed that 

 the Z-chromosome (or X-chromosome, depending upon one's preference) 

 contains genes which are male determining. When only one of these Z-chromo- 



