Doncaster: Determination of Sex 



275 



or a spermatozoon may have one 

 chromosome more or less because it 

 has in it the power of developing into 

 one or the other sex, rather than that 

 the chromosome is the cause of sex. 

 In any case, it is highly probable that 

 sex is not determined simply and im- 

 mediately by the presence or absence 

 of a chromosome. Even if the chromo- 

 some behavior is a necessary link in the 

 chain of causes leading to sex-deter- 

 mination, as seems to the writer 

 probable, it is not the immediate or 

 the only cause of sex." 



If the determination of sex were due 

 to such a simple cause as the law of 

 chance, the two sexes ought to be 

 produced in equal numbers, unless 

 disturbing causes interfere. Every- 

 one knows that in many animals, at 

 least, the sex-ratio is approximately 1 :1, 

 as this hypothesis demands; but when 

 the statistics are large enough, it is in 

 general found that the excess of males 

 is greater than the hypothesis in its 

 simplest form calls for. Worse, the 

 sex-ratio seems to fluctuate according 

 to a variety of conditions. 



"For 100 females born, it has been 

 found that in man the ratio of males 

 averages between 103 and 107, in the 

 rat about 105, in the horse ninety-eight, 

 in the dog about 118, rising in some 

 breeds to over 140, while some animals 

 have an even greater divergence from 

 equality." The ratio is still more 

 upset in man when still-births are 

 counted, for a large part of early births 

 and abortions are those of males, who 

 apparently have a lower vitality than 

 females at that stage of life. Evi- 

 dently, then, the proportion of young 

 of the two sexes produced is not equal, 

 as the chromosome hypothesis seems to 

 demand. 



It has not been difficult, however, to 

 find hypotheses to explain this. Two, 

 at least, can be supported by some 

 facts: "If it is assumed that the sper- 

 matozoon determines sex, the male- 

 producing spermatozoa may perhaps 

 be somewhat more active, or for some 

 other reason, such as slightly smaller 

 size, more successfiol in entering the 

 egg; if, on the other hand, the egg has 

 some share in sex-determination, there 



ma}' be a tendency for the polar divisions 

 to occur rather more often in such a 

 way as to produce eggs of one sex 

 rather than the other. 'Because, there- 

 fore, the sexes arc not exactly equal in 

 number the hypothesis of sex-deter- 

 mination by the germ-cells which unite 

 to form the fertilized egg is not dis- 

 proved. What is required is to 

 investigate the causes which have been 

 found by observation to influence the 

 sex-ratio, and to relate themj, if possible, 

 with chromosome behavior." 



EXTERNAL INFLUENCES 



Such influences are to be found in 

 temperature, nutrition, age of one or 

 both of the parents, moisture content 

 of the germ-cells, age of germ-cells at 

 time of fertilization, hybridity, etc. 

 The experiments and observations on 

 the influence of these various factors are 

 numerous, and most of them treacher- 

 ous. In many cases they can be shown 

 to have no effect on the sex-determina- 

 tion ratio, but merely to produce a 

 differential mortality in the sexes. In 

 other cases, the determination of sex 

 does seem to Dr. Doncaster really to 

 have been influenced. "On the whole, 

 therefore, the study of the sex-ratio, 

 while not leading to any positive con- 

 clusion with regard to sex-determina- 

 tion, makes it necessary to reconsider 

 the simple hypothesis of final deter- 

 mination by one or other of two kinds 

 of germ-cells, to which the facts of 

 sex-limited inheritance and of chromo- 

 some behavior seem naturally to lead." 



Lack of space prevents a review of 

 the interesting chapters on secondar\^ 

 sexual characters, hermaphroditism and 

 gynandromorphism. Dr. Doncaster 's 

 general conclusions must be considered 

 at some length, even though the reader 

 of this review has not been given in 

 manv cases the evidence on which 

 they are based. 



"Beginning with the question of the 

 stage at which sex is determined, it 

 was shown that in some cases it appears 

 to be determined already in the un- 

 fertihzed egg, in other cases to depend 

 on the spermatozoon and to be fixed 

 at fertihzation, and in other cases 

 again to be capable of modification 



