Doncaster: Determination of Sex 



273 



fertilization of the egg, since division 

 of the early embryo always gives rise 

 to individuals of like sex. 



The complicated phenomenon of sex- 

 limited (or sex-linked) inheritance, which 

 has been carefully investigated by many 

 genetists during the last fifteen years, 

 throws further light on the problem of 

 sex-determination. Color-blindness in 

 man is a common example : affected men 

 married to normal women have, as a 

 rule, only normal children; their sons 

 show no tendency to transmit the 

 affection, but some of their sisters and 

 probably all of their daughters may 

 transmit it to their male children. It 

 is thus commonly said "that the defect 

 appears in men and is transmitted by 

 women. It has been possible to deter- 

 mine exactly the proportion of affected 

 individuals likely to appear in a given 

 family ; but in many cases these propor- 

 tions are not found exactly, even in 

 breeding experiments where large num- 

 bers are dealt with. The evidence from 

 sex-limited inheritance can not be 

 reviewed at length here, but it is of 

 such a nature that to Dr. Doncaster 

 "it seems impossible to doubt that sex- 

 determining factors are borne by the 

 ova and spermatozoa, and from the 

 regularity of the observed results, that 

 sex is in general fixed from the moment 

 of fertilization and is not altered by 

 events which may take place later." 

 On the other hand, the occasional 

 upsetting of the expected ratio indicates 

 that other, obscure causes may be 

 sometimes at work. 



EVIDENCE FROM CELL-DIVISION 



The next line of attack which Dr. 

 Doncaster considers is one that has 

 been pursued for only a few years, but 

 which has proved particularly profit- 

 able — namely, the study, microscopic- 

 ally, of the germ-cells at the time of 

 their divisions. The evidence here ob- 

 tainable is based on the behavior of 

 those much-talked-about bodies, the 

 chromosomes. 



"The nucleus of any cell in its 

 ordinary condition is enclosed in a 

 membrane, and consists- of a more solid 

 substance bathed in a fluid. Scattered 



evenly over the network are exceedingly 

 minute granules of the substance known 

 as chromatin which is especially char- 

 acteristic of the nucleus as' distinguished 

 from the cell-protoplasm. When the 

 nucleus is about to divide the chromatin 

 gradually collects into masses, probably 

 by the contraction and concentration of 

 the threads of the network. These 

 chromatin masses are known as chromo- 

 somes, and in general both their number 

 and their relative sizes are constant not 

 only in all the cells of any individual, 

 but in all the individuals of any species 

 Their number varies greatly in differem 

 species, so that the chromosome number 

 may be regarded as a definite specific 

 character." At each cell division, the 

 chromosomes divide in halves, one half 

 of each going into each one of the 

 daughter cells. The nimiber of chromo- 

 somes in the daughter cells is therefore 

 the same as that in the mother cell, 

 being in each case the number char- 

 acteristic of the species. But if a 

 sperm or egg-cell is preparing for 

 conjugation, it goes through a so-called 

 "maturation division" at which the 

 number of chromosomes is reduced to 

 one-half that characterizing the species. 

 When it unites with a cell of the opposite 

 sex, the full complement is thus restored. 

 Obviously, if it were not for such a 

 maturation division, the number of 

 chromosomes in the fertilized egg-cell 

 would be doubled at each generation. 

 Without stopping to discuss the complex 

 details of this cell division, or the action 

 of the chromosomes in parthenogenesis, 

 let us proceed at once to the most 

 interesting part of the subject — the 

 now famous case of the X-chromosome. 

 In some species, one sex has one less 

 chromosome than the other, and this 

 odd chromosome is known as the 

 X-chromosome. The other sex, pos- 

 sessing an even number, is considered 

 to have two X-chromosomes. In the 

 white human race, for example, von 

 Winiwarter found forty-seven chromo- 

 somes in the male, the forty-seventh 

 being the X-chromosome, while the 

 female seemed to have forty-eight, 

 both the forty-seventh and forty-eighth 



