376 INTRODUCTION TO CYTOLOGY 



develop a definite group of characters, including- a particular sex, under 

 a given set of environmental conditions. 



In those organisms possessing heterochromosomes the sex of the 

 individual under all ordinary circumstances is dependent upon the kind 

 of sperm (or, in some cases, the kind of egg) functioning at fertilization, 

 and does not change thereafter. Furthermore, it apparently cannot be 

 changed by many methods commonly supposed to be efficacious in this 

 respect. So far the chromosome theory is valid; but it does not follow 

 from this that the sex which is characterized by a certain physiological 

 state and is correlated with a certain type of chromosome complement 

 and a variety of secondary sexual characters, is so firmly fixed that it 

 cannot be altered by any extraordinary means. The metabolic state, 

 even though its regulation may be accomplished in part through a visible 

 mechanism, is the resultant of a complex series of reactions which may be 

 interfered with at many points. In some cases this metabolic state has 

 been artificially altered to a degree sufficient to bring about an actual 

 reversal of the sex. It is admitted that other heritable characters de- 

 finitely associated with constant genes are greatly modified in the manner 

 and degree of their expression by environmental influences, and the 

 evidence now at hand indicates that no exception to this rule can be made 

 in the case of sex. 



In criticizing the results and interpretations of Riddle, Morgan (1919a) 

 points out that the behavior of a certain sex-linked character worked out 

 by Strong (1912) indicates that the females which were " changed into 

 males" have the male chromosome complement, and that sex is as much 

 a matter of chromosomes here as elsewhere. He declares further that 

 there is as yet no known case in which the sex determined by a chromo- 

 some mechanism has been changed by other agencies in spite of the 

 chromosome arrangement. The evidence here points to the conclusion 

 that when an alteration of the sex is induced, this does not occur with- 

 out a corresponding alteration in the chromosome mechanism. In Droso- 

 phila, however, Sturtevant (1920) finds that the intersexes observed by 

 him are modified females with the usual two X-chromosomes. Here, 

 therefore, certain male characters at least are present in an organism with 

 the female chromosome complement. 



The number of instances of change of sex in forms normally controlled 

 by a chromosome mechanism will probably increase as the nature of 

 sexual differentiation becomes more fully known and as experimental 

 technique improves; but it is also probable that in animals the sex of 

 which we are most desirous of controlling, practical difficulties may pre- 

 vent the attainment of satisfactory results. Slight differences in the 

 responses of the two kinds of male gametes (or female gametes) might 

 conceivably make possible a control over the kind functioning, but it 

 seems more probable that the sex of the individual will be found to be 



