274 



The Journal of Heredity 



being assumed to be X-chromosomcs.' 

 Now the X-chromosomc is cons])icuous 

 because of its unpaired condition, in 

 contrast with all the rest of the chromo- 

 somes in the cell, and it can therefore 

 be followed under the microscope, by 

 anyone possessed of good eyes and 

 unlimited patience. Its behavior has, 

 in the minds of many, gone far to settle 

 the question of sex-determination. 



THE UNPAIRED CHROMOSOME 



"It was known at the close of the 

 last century," Dr. Doncaster reminds 

 us, "that in insects of the order Orthop- 

 tera (grasshoppers, etc.) one chromo- 

 some behaved differently from the others 

 in the development of the spermatozoa, 

 and it was soon found that this chromo- 

 some was un])aircd, and that in conse- 

 quence half of the spermatozoa possessed 

 it and half were without it. Its differ- 

 ence in behavior consists in its remaining 

 as a compact body while the other 

 chromosomes have the form of elongated 

 loops at the stage at which the pairing 

 of the ordinary chromosomes takes 

 place, and it is this difference in be- 

 havior, in addition to its apparent 

 connection with sex-determination, that 

 justifies its designation by a special 

 symbol as the X-chromosome. After 

 the unpaired X-chromosome had been 

 discovered in the males of certain 

 Orthoptcra and Hemiptera (plant-bugs) , 

 it was found that a ]3air of such chromo- 

 somes was present in the females of the 

 same species. Since, therefore, in these 

 forms, the female before the maturation 

 divisions has two X-chromosomcs and 

 the male only one, it follows that after 

 maturation all eggs possess an X-chrom- 

 osome, while half the spermatoza have 

 it and half do not. The eggs which are 

 fertilized by spermatozoa containing X 

 will then give rise to individuals which 

 have two X-ehromo.somcs, and will 

 become females, while those fertilized 

 by spennatozoa without X will develoj) 

 into individuals with only one X, and 

 these will be males." 



If men of science could have stojjped 

 here, wc might think that the problem 



' In the negro race, Guyrr's observations show a double X-chromosomc in the male, with a 

 total of twenty-two chromosomes. The count of lar}i;c nnmliers of such small and elusive bodies 

 is extraordinarily difficult, and results arc not to be accepted too conndetitly. 



of sex was a simple one. But it was 

 soon found that although the condition 

 above described existed in a number of 

 species, conditions quite different were 

 met with in other sj^ecies. Both sexes 

 have two X-chromosomes, in many 

 species; but in many of these it was 

 later found that one of the X-chromo- 

 somes in the male was smaller than, or 

 different from, the other. It was ac- 

 cordingly named the Y-chromosome ; 

 and it was decided that, by the laws 

 of chance, all fertilized eggs would 

 have either the constitution XX or the 

 constitution XY: the former would 

 clearly become females and the latter 

 males. But it has been pointed out 

 above that the conditions in some 

 sjDCcies, particularly insects, show that 

 there must be two kinds of eggs, some 

 male-producing and the others fcmale- 

 ]jroducing. The "XY hypothesis," 

 therefore, which would make the deter- 

 mination of sex wholly a function of the 

 spermatozoon, acting under the law of 

 probability, could not be supposed to 

 have an absolutely universal a]jplica- 

 tion. Further investigation has brought 

 many complications to light, so that 

 sex is not now considered by most 

 students to be universally and abso- 

 lutely dependent on an X-chromosome, 

 although there is a good deal of evidence 

 to show that this unpaired or unequally- 

 ]jaircd chromosome has an improtant 

 role in the transmission of sex-limited 

 characters. 



IMPORT.\NCE OF tHRO.MGSOMES 



The simplicity of the " X-hypothesis" 

 is so attractive that it still has many 

 adherents, but "it cannot be regarded 

 as proved. Many eminent biologists 

 believe that the chromosome behavior 

 is not the cause, but is, .so to sjjcak, a 

 sym]3tom of sex, and that the cause of 

 .sex-determination lies deeper. No one 

 belie\'es that the ])rcsence of horns is 

 the cause of a Red Deer being a stag 

 instead of a hind; horns are a regular 

 accomjianimcnt of maleness in most 

 deer, but are certainly not its cause. 

 So it may be maintained that an egg 



